Delegate one task to multiple people

In my company (a reverse auction website) I need to coordinate various aspects.

One of this is, for example, buy a trip to London.

In the group of "buy products" there are two guys. If I delegate "buy a trip to London" to one person is a lot restrictive. I need that the two guys know that they have to buy the trip whenever they can. And if one can buy it faster can close the task and the other guy can continue with another task.

Sorry for my english.

Thanks to PP's programmers!