On the Pigtronix dual exp pedal, you’ve got a 20K pot where as EHX generally goes by the 10K pots of the M-Audio, Moog EP2, etc. That means either half or about 1/9th (depending on whether the exp pedal’s pot is linear or logarithmic) of the sweep is not usable, and the usable part is either going to be dead on for sweep, slow, or fast (depending on whether the STM is expecting linear/log, and the relationship to the expression pedal’s pot – if they’re both the same, it’ll be spot on, but if STM is linear and the pedal is logarithmic, it’ll be fast; reverse that and it’ll be slow.)

To boot, the TRS jack on the STM may not correspond to how the Ptronx exp pedal is set up, which either means adapter city or just a new expression pedal.

Welcome to the fun world of cussing about passive electronics. Are you handy with a soldering gun at all? a simple adapter could be made to halve the pot in the expression pedal and you wouldn’t need to take anything apart [just need an extra patch cable or a small enclosure and some jacks, really.]