They are different base medications that do exactly the same thing so taking them both won't help "more" than taking one or the other (although Zyrtec seems to be the stronger antihistamine of the two).
Claritin D does contain a decongestant (pseudoephedrine) that you can take separately if you're wanting a decongestant and an antihistamine.
It is possible that you'll have mild side effects from taking them both (dry mouth, drowsiness, etc.), so it's not a good idea to take them both.
