Why aren't tilting propellers used as an alternative for ailerons or elevators?
It isn't done because a moving control surfaces is easier to design and build than an engine mount that rotates. 

Plus the associated structure needed to accommodate the thrust, p-factor, and gyroscopic loads is going to be a lot heavier than ailerons, spoilers, moving tail plane, etc.

Another issue is if one loses one or both engines, your roll and pitch control disappear as well.