Control surfaces are split into separate units to give redundancy. A long range aeroplane needs more backup systems in most areas of the aeroplane. The different control surfaces (there are 4 elevators) are powered from varying hydraulic systems to provide safety in the event of multiple failures.
The ailerons are split into inners/outers (all powered from differnt combinations of the 4 hydraulic systems). At high Mach Nos., the outers would have too much of an effect and cause wing twisting, so they are locked in place whilst the flaps are up and the inner ailerons, which are behind the inboard engine, are the only ones that move. In combination with the upper wing surface spoilers for large control displacements, this is perfectly adequate.
I think the Mach crit for modern jets is about M.93. This training is done on simulator conversions only usually now because of the expense and the fact that reproduction of flight characteristics on them is excellent. The 747-400 is very happy cruising at M.86, although these days this sort of speed is frowned upon.