Catalysis of the reaction: 2 6,7-dimethyl-8-(1-D-ribityl)lumazine + H+ = 5-amino-6-(D-ribitylamino)uracil + riboflavin.
6,7-dimethyl-8-(1-D-ribityl)lumazine:6,7-dimethyl-8-(1-D-ribityl)lumazine 2,3-butanediyltransferase activity