What you do with your game resources - be it actions per round, powers or action points - during a combat is your decision and your decision alone (barring mind-control effectsD&D in general, and particularly 4e, is really not built for Player Character vs. Player Character combat. There are no guidelines for such a fight, so basically, it's up to your DM.
Why you do this?
With the role system, some classes are just going to be innately better at pvp than others.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.