Birchas Kouhanim - do all Kohanim say the brocho before?

According to minhag Ashkenaz, before Birchas Kouhanim, (if there is more than one Kouhain) does only one Kouhain recite the brocho "...asher kid'shonu b'k'dushosou shel Aharoun..." and be moutzi the other Kouhanim, or do all the Kouhanim duchening, recite it at one time?

Rallis