I did a little bench test of my 3500EF, 3500EM, ARINC, Magnetometer, and 496 GPS. No real issues, but I noticed the same thing with the knob on both displays - most of the time the first "click" is ignored.

I had no problem with the 496 - it was hooked up to serial1 of the 3500EM and I was able to get its information on both displays (as GPS2). It was neat to put a flight plan into the 496 and see the fpln "crossfill" onto both displays.