In regards to the 2nd player controls, I was aware of this issue when implementing them, I had to design it in such a way where in the event both players do not own a controller, they could fallback to using the keyboard, additionally I did not want to put the players in a situation where they needed to press a key on the others finger space, I am very aware of the problem and intend to edit the controls to come out in a more comfortable way, additionally maybe adding a way to rebind them.
In regards to the cards, I am unsure if the issue you were having was regarding using the cards, or with being able to play them, just in case I will cover both cases.
Cards activate automatically, and do not require any additional input from the player after placing them on the field, they mainly influence what enemies the other player will fight, however most of the effect cards will affect the player placing them(with the exception of the ? card.)
Cards also have a cost, indicated by the number of diamonds on the top left corner of the card(1~3), and if you look on the side of your playfield you will see 2 numbers, this shows how many points you used, and what the max points you can use between all the cards on your field(used points / max points) each round the max points will increase by 1 up to it reaching 15, allowing you to play more cards, you can also replace active cards to basically refund the cost for them that will factor into the new cards cost.
I may include a visual illustration in the next update specifically pointing the above out as even outside this jam when showing friends they had trouble picking up on that.
In any case, thank you for the feedback, I definitely need to go over the controls stuff some more as that seems to be the common trend of the feedback so far, and thank you for checking out the game!