Certain setups can compliment each other - however, the biggest thing to be weary of in using such multiple-detector setups is to insure that one does not interfere with the other.
There's really no good way to be able to tell if such interference is occurring, and the
GOL page here well describes why:
Radar Detector Tests and Reviews by Guys of LIDAR
Truthfully, most who decide to use such dual-detector setups typically go with a remote-mounted detector, such as the old Escort SR-series or the current 9500ci/STi-R, and pair it with an in-cabin V1, or, alternatively, select the Bel STi-Driver to compliment a V1 as an in-cabin, dual, setup.
You can easily see where the performance/features of these detectors "mesh" with one another, and serve as good compliments.
With the former forward-of-firewall and in-cabin separation, most agree that any potential interference *should* be minimal.
With the latter, although it would make sense that the V1 would be shielded from any potential effects from the nearby STi-Driver, no-one is really sure of the converse of that...whether or not the STi-Driver will somehow be affected by the V1.
The purpose of multiple detectors 
Linear Mode
