Welcome SandyGirl and SheFly!
Sandy: You could just put some slicker tires on your MTB and it would help make the street riding smoother. Although I subscribe to the premise that one can Never have too many bikes - they do start to whine just a little when I walk by one that hasn't been ridden in awhile (like my MTB! LOL)
SheFly: TT on a tandem?? Now ThAT sounds like a blast!!! I would Love to see a pix of your TT Tandem!
Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways, champagne in one hand, strawberries in the other, body thoroughly used up, totally worn out and screaming: "Yeah Baby! What a Ride!"