But if the plane was designed to have one servo mounted at the center of the wing, mounting two there isn't a problem.

Don't bother trying to add new locations outboard in the wing - just open up the existing servo hole at the wing center section, make sure you have hard points, and mount both servos next to each other. I have done this many times.

The Flutter issue is a different animal. Nothing you are doing is going to make it worse. (if it will even be encountered at all.) Now you have aileron redundancy and you can also play with flaperons, spoilerons, aileron differential and individual subtrims. Many will scream at the idea of using flaperons and spoilerons combined with strip ailerons, but I have been enjoying them on my trainers and sticks for as long as Dr. Rick has been stuck in the Land of the Lost.

Kurt