"both games per opponent start immediately"

How bad is it to put our opponent playing aganst himselfe ? I mean, he plays e.4 and I g and play e.4 as well in the game I'm white and I keep going like that ..

How bad is it !? Is there something that he can do against it ?

Yes, he can report it to the staff and they will require you to stop this practice.  The staff does not make a big announcement about not doing this but they do consider it a form of cheating and will treat it as such.

There was a player rated about 1300 and relayed moves between two players rated near or above 2000. When discovered, he was forced to resign the games. It's against the rules, and frankly a sign of mental weakness.

I assume that you would then wait until he responds as black before you respond as black, but you will have to time out before him if he waits you out.