Question 1, are you able to use more than one enhancement for an attack?

EX: Vet Diablo with 3 arrows has psylocke and a Exp Taskmaster on his enhancement click next to him. Does he recieve the bonus from one of them or both?

Question 2, are you able to use enhancement for more than one person a turn? I think you are able to do so, but my friend says it is only once per turn.

Each person with enhancement can increase the damage dealt by the attack. So, both Psylocke and Taskmaster could each increase the damage dealt by 1 (for a total of 2).

You are correct, one character can use enhancement for several attacks in one turn.