2 ways

Strategy 1: Send email or phone to negotiate Offer 1(i.e ask more money),so he can have time to look into Offer 2. Do negotiation is part of hiring business process so don't worry.

2) Accept Offer 1, when Offer 2 comes, if it's good. Accept it and rejects Offer 1. Northing hurts. Acceptance is by no means you must go with it.