RE: Coreless servo for dual servo elevator
Typically when a servo fails it either jams or it hardovers it does not just "let go". Either of these would cause you to lose your elevator regardless if you had another servo installed.
Not sure how you'de split a P-38 Elevator...
If you need to use two servos, align them like we align multi-servo ailerons using the programmer or matchbox and don't worry about it.. use a good cored motor digital to keep the cost down and you'll be fine.
DP