With mismatched motors on the same outlet (through a Y-cord custom multi-outlet splitter box) the speed control circuit can get confusing signals sent back, resulting in the the circuit just giving up and running all the fans on that outlet at full speed. Once the control goes to zero speed (off) on the convection fan outlet then all the motors on the outlet will turn off, but when it tries to turn them on again, if the same odd back feed from the motors occurs, the fans will jump back to full speed even though the control is trying to run them at a slower speed.
So two rules:
1) Keep total current draw on the Convection outlet to 4A or less.
2) Use matched motors if possible. If they aren't matched you may still be okay, but you may well run into the full speed (when trying to run at slower speed) effect I described above. If this occurs you either live with it or you have to get the non-matched motor off the splitter.