I question why you would go through all the pain and suffering to make both flaps work together, especially at both ends, using belcranks.

Two mini servos and either mixing or a JR Matchbox will make it so much easier to do in a lot less time. You can adjust the end points on both servos/flaps individually. If you end up with a little roll from extended flaps, you can easily change the flap position to fix it.