About trading cards.

No, you can only play one game at a time if you want cards. You can't play two games while idling.

EDIT: After my small search(and after this all these replies) I concluded that I was wrong. It drops, but a slower rate. Either way I think it's better running one at a time.

I'm sorry for the mislead, I was too conviced of myself :P

You will receive cards from both games but when multiple games are launched it slows it down a lot for the rate at which you receive cards.

Also note that you need to have 2 hours playtime on a game before cards are dropped from the game :)

I recommend using IdleMaster, game needs to be played for 2 hours before it starts dropping cards. If you already played 2 games after 2 hour mark and idle them together you will get drops much slower so try idling 1 by 1.

One strategy is to launch two (or more!) games for those first two hours, then after 2 hours, just run one at a time.
But as others said, you'll still get the drops, just more slowly. :D

afaik you get cards based on what is launched first / which ever one gets displayed in the friendslist. if you have recieved a bunch of cards recently there is like a cooldown period where you cant have drops.
nothing is for sure but thats what i observed