I'm using ltc rabbit and wondering if I can gpu mine and cpu mine with the same rig?
CPU mining generates a lot of heat and shortens your lifespan of the rig. It is possible but you are adding extra heat to the rig without actually gaining a lot more hashrates. Also, consider buying a scrypt ASIC, GPU mining would not be profitable very soon.
CPU mining may not be profitable when compared to GPU mining assuming the coin you are mining can be mined using both CPU & GPU, but to say that CPU mining generates a lot of heat and shortens the lifespan of a rig is insane! Unless the cooler on your CPU is faulty, or the thermal paste is dry, the CPU shouldn't overheat in the first place. That is a misconception.