Decorate with white Christmas lights, and buy them after Christmas, or the day after Thanksgiving... you can do different things with them, but still use them for both receptions. For mine we strung them across the gym and it makes a nice false ceiling, you can also drape fabric through them to make it more enclosed.
I've seen this done aan it can look awesome. You can ask neighbors to borrow--just make sure you label and be prepared to replace them if something happens.