I can't imagine they would retract their initial job offer if the interview for the other job goes well? Is the other job one you would prefer to have or have more prospects? They might view you as unambitious if you don't go for it without a valid reason.
I say, keep in mind they offered you this opportunity - so they obviously think you have the potential to succeed in it! Be confident in yourself, and remember, you don't get many great opportunities handed to you in life, so take advantage when you do and go fot it. Good luck!