2 steel legion Chimera on the same planet, would they each prevent 1 damage per attack? would I prevent 2 damage per attack

Each separate, individual reaction ability can be used 1 time per triggering condition. So you may use both copies of the unit separately. The are considered separate, individual reaction abilities because they are on different pieces of cardboard. The fact that the title of the card is the same does not stop you from using each copy 1 time for a given triggering condition.

So you would trigger each one, and each one triggered prevents 1 damage.

FAQ talks about this.