I suppose it all depends on how you want to report it to your client. If you are just looking to bill a larger amount, you could create a Task and assign it the higher rate. The "Rates" work in the following order: Global Rate -> Client -> Project -> Task -> Custom. So the Task is second to last in the assignment and will override other rates. So say if you normally have a client rate of $50, and for a Task that has two people you could assign the Task Rate to $100.