Answered by Angie, Hiring Expert at Asurion, on Tuesday, April 21, 2015
Congratulations on being considered by multiple employers! In this scenario, your best bet is to be 100% transparent with both organizations.
Most employers will expect a response to an offer within a couple of days. I would suggest letting the second potential employer know that you have an offer that you must respond to, and see if there is anything they can do to expedite the selection / interview process on their end to accommodate your deadline. If they can, then great! If they cannot, then you will have to make a decision to either accept the offer in hand or decline it in the hopes of getting an offer from the other employer in the future.
Best of luck!