a 5'-end (N(2),N(7)-dimethyl 5'-triphosphoguanosine)-ribonucleoside in snoRNA + S-adenosyl-L-methionine <=> a 5'-end (N(2),N(2),N(7)-trimethyl 5'-triphosphoguanosine)-ribonucleoside in snoRNA + S-adenosyl-L-homocysteine + H(+)