Hi Donalf,
I’m not sure exactly what’s going wrong for you, but I just tested changing a velocity variable by setting it to arbitrary values (0 and 120) with keypress events, and Modular Synth responded as I’d expect. Are your two synths definitely independent of one another? Not sharing some var or event that’s causing a conflict? And are you certain that each synth is playing the notes you think it’s playing? It’s not completely clear to me which is doing what in your vid - maybe running up some distinctly different synth presets, and perhaps hard-panning each to left or right while you’re testing, will help troubleshoot that. Or just drop one an octave, maybe.
Hope you get it figured out - curious to hear either way!