Catalysis of the reaction: 2 1-O-sinapoyl-beta-D-glucose = 1,2-di-O-sinapoyl-beta-D-glucose + D-glucose.
1-O-(4-hydroxy-3,5-dimethoxycinnamoyl)-beta-D-glucoside:1-O-(4-hydroxy-3,5-dimethoxycinnamoyl-beta-D-glucoside 1-O-sinapoyltransferase activity