A correspondent writes to say that someone corrected him when he asked (before a tennis match): Who are playing? His friend said it should be Who is playing? He agreed that the plural sounded funny, but couldn't think why.
The plural sounds a bit odd because it is the normal unmarked use of who to be singular. This is because the who question is usually definite and particular. Who wants to open the bidding? is asking a specific individual to begin. Note the contrast with the indefinite and general use of who in the singular, as in Who wants to get Alzheimer's?, which means 'There is no-one who wants...' Faced with a question where a singular or a plural answer are both possible (as in the tennis case), people will find the singular interpretation more natural than the plural.
A plural indefinite who is perfectly possible, however, especially when the post-verbal element is explicitly multiple, as these examples illustrate:
Who are our top ten salesmen this year?
Who won first three places in the race?
Who are playing Smith and Brown in the doubles?
Who were fighting to preserve their nation?
Who are playing Hamlet and Horatio?
The last type of case is nice: it would be impossible to have a definite singular interpretation here, unless the actor somehow managed to play both parts simultaneously!