cant ping when adding secondary ip to 1 fastethernet interface on 1700 router

I am testing on a local router adding a second ip. when i add a secondary ip in the same subnet everything works but when i add a secondary ip in a different subnet i cant ping from or to the router. i also try adding a second ip route 0.0.0.0 0.0.0.0 mynewgateway but that did not work. I will change providers soon and i would like to make sure that when they put the router in the new location everything will continue working. can this be done? thanks