If you store different prescription drugs in one container, could there be a chemical change?

Question

Will it affect the expiration date of each medicine?

Answer 1

It would depend on the medicines.  If they are both coated tablets of stuff that doesn't have a significant vapor pressure, then they're not likely to interact.  Expiration dates should not be affected under those conditions.

It would be much wiser to store them in different containers, labeled with what they are - especially if they look at all similar.  

Disclaimer:  I am not a pharmacist.  Your best bet is to call one of those and ask.