I would suggest using midi over trying to split the cables. I use an expression pedal and an aux switch with a PF and an MF and it was cake to get them synced up (though I am having a slight issue with the PF not saving xmt cc settings…).
The instructional videos provide a very nice, step-by-step process to doing this. The short of it is you figure out which pedal you want as the master, set midi transmit to 'on', then select which channels tip, ring, and t+r transmit on. You then go into the other pedal and select what functions you want those channels to control. The expression pedal will be on the same channel by default.
