Unless your supply is way up on a hill and your house is at the bottom, the supply pressure is the max you'd ever see in the house.
To run the water probably about 12-15' from one end of the 8' shower to the other, the temperatures wouldn't (at least start out) the same since you'd have that long of a run of pipe to warm up. Plus, any changes you made would be slowed by 2-3 seconds as the changed mix made its way to the further showerhead. Plus, how often is it that two people like the same exact temp for their shower? Slim chances...put in two controls, and the controversy goes away.
The trims are fairly large to allow for maintenance and someone's idea of what looks good (maybe not yours!). Many of the handles are long loops or levers, and the diameter of the trim is larger. If you were to choose simply a round handle, in theory, it could be smaller, but most don't spend the money to make multiples.