Returns a new CompletionStage that, when this and the other given stage both complete normally, is executed using this stage's default asynchronous execution facility, with the two results as arguments to the supplied action. See the CompletionStage documentation for rules covering exceptional completion.

Parameters:

other		the other CompletionStage
action		the action to perform before completing the returned CompletionStage
<U>		the type of the other CompletionStage's result

Returns:  the new CompletionStage