OK yes I can log in, but what if I want to play a game looking at both sides?

Is there a Hot Seat option? or do i have to create a second account and log in and accept my own challege?

Thank you,

Hello and a quick thank you for the great game. My friend and I (both purchased the game) have finished the tutorial and now wish to play Multiplayer with each other. But it appears I need a serial key to do so. But I purchased the entire game on Steam and therefore have no serial number...or is it ...