Zynthian run only one instance of ZynAddSubFX. All the ZASFX layers are executed by this single instance, as different “parts”.
Currently, Zynthian doesn’t support loading of parameters (xmz) files. Only instrument files (xiz) are supported right now. Anyway, ZynAddSubFX has a very powerful OSC interface that can be used for controlling almost every aspect/feature of ZASFX, so please, open an “issue” in the github repository and i will see how to integrate this feature. I also think that could be an interesting add