RE: Second rifle choice?
I really see no need to get anything that close to the .30-06 for the animals you have listed.
Also, I would not recommend sharing a load between two rifles. What works in one might not work in the other, and at worst, can be very dangerous.
I would go with something in the .25-06, 6.5x55, .260 Rem., or something along those lines, to use for the antelope, but that could also double for a varmint rig. Those may even be too big for varmints, so something along the lines of a .243 Winchester might work good too. Or, if you really want something for just varminting, then the .223 Rem., 22-250, or 204 Ruger would all fill that bill nicely. The .30-06 will work fine for the elk, so I really see no need to go any larger.