Depends how much you want either or both of these offers, and how serious the first one was about working with you-- were they saying you would get a gig but not sure which date/time, or were they saying they were considering you?

If you want to risk losing the new offer, you can ask them to wait while you check with the tentative one. But what I would do, personally, is take the firm offer as soon as it's offered, and then notify the tentative one that you have not been able to hold that portion of the time-- "since I had not heard from you I accepted this offer" kind of thing-- then let them know you are still interested in working with them if your scheduling will permit. They need to understand that a busy working professional can't reserve time indefinitely, and you need to plan to communicate with potential bookers how far in advance you generally try to get your bookings confirmed.

~Susan