If you are going to buy a new bike either way then buy one for 2 up and solo the SM. I never found my SM lacking for 2 up but I come from riding a 66 TR6C chopper with a hard tail frame and long springer for 2 up. Never wanted another until we both put on weight and now the SM handels us both but I never ride 2 up anymore since we got her her own bike.

I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!