I used to do the two camera switch too. My Nikon f401 metered for my Yashica TLR. Works fine, just a little testing to find what works best for you. For me I prefered just a bit more exposure on the TLR then what the Nikon reported. I guess this is exactly what BobF is saying about metering meathods of cameras. (there were other complications for me like the shutter speeds on the TLR were different than standard camera settings: 1/5 sec, 1/10 sec, 1/25 sec.)
To find the best results I metered with Nikon, then bracketed on the Yashica. 1 stop over, 1 stop under, you know...

In the end I bought a handheld light meter. I find it is more convient to use, and won't go back to two camera metering unless forced. Carrying one less camera (and accessories for said camera) means more room for film and is supposed to mean less weight, but never seems to work out that way.