Are you getting EXACTLY 4 chuffs per wheel revolution? That's one way to know that the soundset is correct for the motor and gearing of the engine. In conventional you should get exactly 4 chuffs whether speed control is on or off.
If you have two PS/2 engines (whether converted PS/1 engines or out-of-box PS/2), IF the soundsets are correct, they should run at approximately the same speed in conventional with speed control ON. This was done to allow different PS2 and PS3 engines to run in a MU configuration when in conventional. In other words, for the same track voltage, PS2 (and PS3) engines should run at the approximately the same speed with speed control ON.
Since your two PS/2 engines are apparently running at noticeably different speeds in conventional with speed control ON, I'm thinking you might have one of those odd-ball motor-gear configurations that GGG mentions. Perhaps MTH did not issue the "correct" soundset for that engine.
If you then tinker with the number of stripes to get the converted PS1 engine (with incorrect soundset) to run at the desired speeds in conventional, I believe that engine will not run at the correct scale speed when you then convert to command mode. Of course once you figure out how to print/apply your own tach stripes, it only takes a few minutes to change tach tape. Perhaps that's the way to look at the predicament.
