Is it possible for two miner softwares to be running on a machine? One doing CPU mining, other doing GPU mining?
Yes, sure.
You can also use one runing software and one runing software different instance (same or differents mining software) for each your GPU.
If you have 6 GPUs, you can run 1 (CPU) + 6 (GPUs) = 7 instances of mining software on the same machine.