There is no limit to the number of MIDI tracks that can drive a single synthesizer. However, you have to keep in mind that
all events on a common MIDI channel are going to be combined into one data stream to the synth. It can work no other way, unless the synth is multi-timbral and the tracks are on separate channels, in which case the synth is effectively serving as multiple independent synthesizers.
When splitting out a portion of a MIDI track into multiple tracks, it's important that the affected portions of the original are muted or slip-edited so that they're not simultaneously sending MIDI data in competition with the new tracks. That includes continuous-controller envelopes! If you're using volume automation, for example, put a jump segment in the original track with no automation nodes within the moved section.