Poor College Kids road race has multiple Cat 5 races.

5A is at 9am
5C is at 11am.

Is it OK to sign up and race both?
The race is just about 30 miles so it shouldn't be problem to finish the first and enter the second race.
If yes, do I get two credits towards the Cat 4 upgrade?

How about for those eligible to race lets say Cat 3 & Master 35+? Can they enter two races of the same event?

Thanks

J.