I have a crappy PC and a very powerful one. My girlfriend and I would like to play Overwatch together, I know I can stream non-steam games, but can I stream the game to the crappy computer via steam's in-home streaming while playing it ?
Do I need to create another steam account to do it ?
You probably can, but that won't play 2 copies of the game.
Game streaming just runs the game on your PC, send the video to the receiver and controller input back to the PC. You could at most play the same character.
You do not need a second Steam account to stream on a second computer, you just need to log in on steam on both computer. In your library you will have the option to stream a game instead of playing it or installing it (you will still have the option to install it with the dropdown menu).
You will be able to stream to your other computer and play the game from it (or just let someone watch), but thats it.
In the case of Overwatch, I do not recommend it since you will add delay between the game server and you since you need to relay data to one more computer.
