Can somebody help me to make my XP-80 and Sony MDM-X4 (minidisc multitrack recorder/mixer) work in sync? I checked both manuals; neither was explicit enough to make this work (but I still hope it can be done).

Here's what I want to do: set up time signature on MDM-X4 (i.e. lay a time/tempo track) (and even pre-record a part on it) (done). Then record something on XP-80 sequencer with the same time signature (done). Then dump/record it to MDM-X4 so that the new part will be correctly overdubbed on the prerecorded track on MDM-X4? Or even just make them both play their parts in sync? I used a pair of MIDI cables, tried to use all options: MMC, MTC etc., tried to use either device as a master/slave, but at best got one starting with a delay vs. the other. Any general advice or more specific to my situation will be much appreciated.