It's right before the battle for Hoover Dam, and Boone and the Courier get some action right beforehand- after all, they don't know if they'll both make it. Can have whatever set up you want plot line wise, (heat of the moment, already in a relationship, whatever floats your boat) but preferably good/ncr-ending courier.

Bonus points: They get caught.

Super bonus points: At the end, one of them gets badly injured, but eventually makes it /sucker for happy endings.