Need some help with Archisteamfarm.

Yeah, ASF simulates you playing the game to get the card drop, it does one game at a time to do this. Steam only allows you to play one game at a time, so ASF has to wait until you're done playing to resume farming.

Idlemaster still works. Have it installed and use it when you want to play a game at the same time as idling. When you're done playing, switch back to ASF. Simple.

As for idling mutliples, it doesn't work well, there is no advantage to it.

The reason why you cannot play CS:GO is because ASF is independent and does not trick the Steam client you're using into thinking you are running the game on your computer. It is another client idling your game, and each account's library can only be used by one client/PC at a time.

Does anyone know how I can play CSGO while farming cards?

You could be playing several other titles in offline mode, but if you want to play CS:GO online, then answer is simple - you can't do that with ASF, for reasons @InuKaT stated above, together with my reply. And if you truly understand the answer, you'll be actually very grateful that ASF doesn't allow you to.

And how to I farm multiple games per account?

You don't, ASF knows better than you how to farm the most effectively. You can, however, provide some crucial variables for ASF which can't be guessed neither checked from the data ASF has access to, as described in Performance - this will affect chosen cards farming algorithm, as well as other logic.

All my games have more than 2 hours already but it seems ASF farms 1 game at a time per account.

Correct, even Complex algorithm will do that. As I said, ASF knows better. The more you dig inside, the more time you'll waste and the more you'll see how wrong you are. You can still farm in manual mode through !play command, but that will never be more efficient than ASF automatic mode, regardless what you do. Reading the wiki page I linked you above should answer why, but if you have any specific questions after reading that, feel free to ask - I'll answer and add to FAQ/page if needed.