I was going to suggest using both, but I saw, as I was scrolling that 'catspaw49' had it. Use both harp players, swapping licks, as in a duel, throw in some comedy, by having one, out of breath, and falling dead, with the one surviving 'smoking it' on the way out. Your friend might accommodate, not even knowing, other than his part in the piece, that he's being cast, as the lesser...so he may not even be offended...just so long as he gets a part. And besides the possibilities are open for some slick harmonizing that will smoke the competition to a crisp. Good luck..and break a leg!
P.S. there was an old group called 'The Harmonicats' you might be able to find, maybe on 'youtube', or somewhere...a group comprised only of different harmonicas, for input! Ya' might just slay 'em!!