My 57 R-50 ISDT has a single R-51 muffler and works fine. only 250cc firing at any time. My R-75 ISDT has a 1-1/2" pipe into and out of the muffler, same deal, 375cc per cylinder. The 2 into 1 y pipe is a full foot forward of the muffler. BSA and Triumph twins used a single standard muffler with 2 into1 exhausts.