If you have over 2550 Hit, you're crossing the Hit-cap.
If you have over 2550 Exp, you're crossing the Expertise-cap.
Having more of one doesn't give you some of the other. Why should it?
If your class/spec requires both to be capped, look up your stats in your character screen. Both Expertise and Hit are capped at 7.5%. If you have more than 7.5% in either one, you're over-capping and essentially wasting stats. If you're below the caps, you should generally add some of said stat in order to reach the cap.
LONG STORY SHORT:
- Expertise doesn't stack with Hit and vice versa
- Both stats are completely separate
- Having both capped individually is the only way to go
The only exception to this rule (to my knowledge) is how some classes convert Spirit into Hit and how Spell Hit is calculated, as mentioned above.