He is some info from the Guys of LIDAR.It's from the website.I know it's old, but I wanted to post it for anyone new who doesn't know about it yet.
Two Radar Detectors in the Same Vehicle?
Many people familiar with radar detectors know that false alerts can sometimes be caused by other radar detectors operating nearby. But just because two radar detectors do not cause alerts on each other mean that they are not interfering with each other. Here are a few reasons why you shouldn't run two radar detectors together in the same vehicle.
It can "Fool" the false alert filtering
"Detector A" sees the police radar and also sees signals which leak from the other detector in the vehicle, "Detector B". Detector A thinks that the police radar is also leakage from Detector B, and tries to suppress the alert. This results in diminished range or no alert at all from Detector A.
Radar detectors "sweep" the radar bands for police radar. But what is not so obvious is that during their regular sweep, the detectors are also scanning for other non-police radar frequencies that, if detected, could block or prevent an alert. This is due to filtering techniques which attempt to prevent Ka falsies from other detectors. These techniques work wonders in normal situations. But with another detector operating close by, range can be reduced or alerts can be completely blocked.
Superhetrodyne radar detectors contain "Local Oscillators" (or LO for short). LOs create a radar signal that is utilized to "mix down" the incoming radar signal for detection. Although the LO is generated inside the detector, in most cases the LO is "leaked" or "transmitted" from the radar detectors. Many detectors on the road have a 1st LO that operates in the 11-12 GHz range. The problem is, that these LOs also produce "harmonics" at multiples of the LO frequency. So, in once classic example, an LO operating at 11.558 GHz would produce a harmonic at 34.674 GHz, well within the valid bandpass for a Stalker Ka radar unit.
Of course, this poses a problem for detector manufacturers: they must filter the Ka false alerts caused by these other detectors on the road, while still providing superior protection against Ka radar.
Luckily, they have a novel technique at their disposal. In the example above, not only is there a Ka signal produced at 34.674 GHz, but because the Ka signal is being produced by oscillator harmonics, there will also be signals present at the oscillator's fundamental frequency of 11.558 GHz and the second harmonic at 23.116 GHz. These signals would not be present with real Ka police radar since it transmits Ka directly. So if the detector sees a signal around 34.674 GHz, the detector might also look for signals around 11.558 GHz, or the second harmonic at 23.116 Ghz. If one or both of these were present, the detector would know that the Ka radar was a false harmonic from another detector and not police radar, and can suppress the alert as a false.
For example, if there is any interference seen in the 11 GHz and/or 22 GHz range, some detectors might "lock out" Ka band or a section of Ka band for a certain duration in order to prevent a false alert from another detector in the area.
The whole technique gets pretty complex:
-the 11.558 GHz scenario mentioned above is only one example. There are several frequency schemes used by different detectors, that detector manufacturers must account for in order to prevent K/Ka false alerts from other detectors.
-the oscillators in the detectors are sweeping
-the oscillators in some detectors will "park" at different points throughout their sweep
One other technique that is sometimes used, is the detector will look for brief recurring "blips" of radar at certain frequencies which are indicative of the sweeping oscillator of another detector. When the detector sees these blips, it might "lock out" or raise the threshold for a section of K or Ka band for a certain duration in order to prevent false alerts from the nearby detector. Of course, if this other detector is in the same vehicle, then these blips never go away, and sensitivity is never restored as long as the interfering signal is present.
A detailed description of the methods used to filter alerts from other detectors is beyond the scope of this article. Hopefully the above examples are enough to illustrate how running two detectors in the same vehicle can be a problem in light of the techniques used.
It can cause a detector to "park" often during it's sweep or switch to a "slow scan"
"Detector A" is kept busy analyzing and rejecting false alerts due to leakage from "Detector B", instead of looking for police radar. This results in diminished performance.
Some detectors operate by sweeping quickly until they see a radar signal, then they "park" their LO or switch to a "slow scan" to get a closer look at the signal, in an attempt to determine if it is really police radar. With another detector operating in close proximity, it might see leaked oscillator interference from the other detector and "park" or "slow scan" in order to analyze them. Even if the detector determines that it isn't police radar and does not trigger an alert, this will still have the effect of unnecessarily slowing down the overall sweep, reducing effectiveness against real police radar. This can be especially critical to performance against weak instant-on at a distance, or POP.
It could raise the "noise floor"
With microwave oscillators operating in close proximity, it has the potential to raise the "noise floor" in the radar bands. Modern DSP detectors use averaging of multiple sweep samples in order to reduce the noise floor for maximum sensitivity. Raising the noise floor would have the effect of making it so that a stronger signal would be necessary for the detector to be able to pick out radar signals from the noise.
But I've ran two detectors together before and they seemed OK. I didn't notice any difference.
Sometimes two detectors running in the same vehicle will operate quite normally and there is no problem at all, at least part of the time. It might be OK on some bands or frequencies, but problematic on others. It might work just fine, except for certain times when the sweeps of the two detectors coincide with each other in a certain way. And, there's no way to test this, or to "try it out and see if they interfere" because you just don't know what the detectors are doing internally.
The bottom line: there's no way to be sure when they might be interfering or when they're not.
What about the BEL STi Driver?
People frequently ask the question: since the BEL STi Driver does not "leak" then it shouldn't interfere with another detector, right? Actually, the BEL STi does technically still "leak". However, the leakage is so low that there is no detection from today's RDDs (Spectre).
Consider the following facts:
Compared with today's high-end radar detectors, Spectre isn't all that sensitive: it uses the same receiver as cheap "Quintezz" radar detectors sold overseas.
The normal operating range for Spectre is well beyond the separation you would have when operating two detectors in the same vehicle.
So, could there be a problem with operating a radar detector which is more sensitive than Spectre at a distance which is much closer to the STi than the normal operating range for Spectre? We have posed this question to some of the brightest minds in the industry, but there's no clear-cut yes or no answer. The STi still has the potential to interfere.
Can other detectors interfere with the STi? Definitely.
My recommendation: don't run two detectors in the same vehicle if you are depending on them for protection. As for comparing performance between two detectors: any results are always going to be questionable if they are obtained when running two detectors together. For unquestionable results, test each detector individually against the radar source. After that, if you want to run both together for demo purposes, go for it. If the results happen to be significantly different than when the detectors were run individually, you'll know why.