Currently running a tandem for the first time, and occasionally I will have to bump a RP up to a Tandem B spot.  When you switch that pitcher up, he is given a 40 pitch count, which is fine.  However, when you switch him back to a RP, he keeps the same 40 pitch count.  I seem to recall in SimLeague, if you send a guy up to a starters role he will have one pitch count, and if you send him back, he will go back to his original RP pitch count.  I think that way would work much better for HBD.