I have a RB 2011with wireless AP. I configured a second wireless interface (a virtual AP) for public and private network access. I wanted to know if there would be any advantage to putting the two wireless interfaces on separate wireless channels ( i.e: 1 and 5 or 1 and 11). would this reduce possible interference, or does interference only occur with a different AP transmitter?

You can't set your VirtualAP to use a different frequency than what the master interface is using.