I could see getting two similar bikes as long as you could re-purpose one of them. Does your aluminum/rival bike have eyelets for fenders and such? If so, you could use it as your winter training/rain bike. Then the carbon/105 bike becomes your main bike. Do you plan to race, the aluminum bike could be used for that without much fear of messing the frame up in a crash.

If both bikes would basically serve the same exact purpose, then as much as I wanna say you should always own n+1 bikes, I'd say save your money and put it towards some good upgrades or a truely different purpose bike. As other posters have mentioned, a cyclocross bike is a good versatile machine.