All advise good so far, but I have a comment and a suggestion...

The comment - I installed tail servos on a few of my Ultra Sports and all I did was to add lite ply under the surface balsa and harden the holes with thin CA.

A suggestion: Make sure both servos will FIT side by side in the tail before you make any cuts! That is, make sure that once they are in place, the bottoms won't touch preventing the second one from seating correctly because it's hitting the first.