Hi,
Is it possible with al minable altcoins to do CPU and GPU mining at the same time or doesnt it have effect?
Yes it is, for example, XMR-Stack can mine using both CPU & GPU, or you could use a combination of miners, for example, Claymore and CCMiner.
Also, can I mine 2 different coins on the same miner? (pc)
For example: Zcash and Xmr
Yes, this is also possible with some miners, for example, you can do ETH & DCR with Claymore on AMD RX range of cards, I've not specifically tried XMR & Zcash, but if the miner supports the algorithm(s) and also support dual mining, then you could test/try it out.