What you want to do is definitely possible. I am using this quite often.
Once you have added the NIC and set up its ip addressing and routing you will then need to use the IP FIREWALL MANGLE to mark the traffic.
So, because you want to use each public interface for different types of traffic you must mark that traffic accordingly.
Say makr VOIP Traffic, and then mark ALL OTHER traffic.
Then you will use policy based routing to direct VOIP traffic to one interface and ALL OTHER traffic to the other interface.
Dont forget to set up your nat properly though.
Take a look at the wiki for examples: http://wiki.mikrotik.com/wiki/Policy_Ro ... erOS_2.9.x