All the adding happens at once - like in a real computer.
Add the two registers together and store the result in the lower left one. If the result is 8 or more release the blue ball into the black collector. You may only launch one red ball, but you may use two extra balls which start balanced on pieces on the board.
Hint 1: use one of the ball release mechanisms from my other posts.
Hint 2: half the challenge is making sure that the balls don’t collide - you can use unconnected gear bits as faster green ramps, as long as they are only traversed once.