Whether one person does it or more doesn't matter much to me, what should matter is consistency. Which is a lot easier if one person's doing it, obviously. (But if you look at Mega Man: The Wily Wars, the two Oracle GBC Zelda games, Super Mario Bros. 3, and the topic about Castlevania: Symphony Of The Night, you can see it is possible if there is room for discussion.)
Right now we've got a number of Yoshi's Island maps but they're from different mappers. We've got some broken up in pieces and some as a single image per stage, some that are JPEGs and some that are PNGs, some that don't have backgrounds and some that do...
(Ideally they should be single-image PNGs with backgrounds...hint hint...)
Then you have things like whether or not or how exactly to place enemies and/or items and/or dynamic elements like moving platforms, whether the "blank"/"unreachable" areas are filled in and how, how the "rooms" are arranged and connected (and if they fit on a grid), what background colour is used for any non-gameplay/"outside" area of the image, how copyright/title/etc. information is presented if they are at all...so on and so forth.