Sorry to double post but I tinkered about for a bit and I figured it out and so I shall post it here.
I'm going to use my drum machine in the example because it's the only one I've ever used. Each pad of the Electribe is assigned to a channel, I think that is a pretty basic feature for drum machines. Seems like the maximum channels for most sequencers is gonna be 16.
What I have done is made the Electribe the master, and selected slave mode on the XW P1. But I plugged the midi out from the XW P1 to the Electribe so I can control the oscillators with the XW P1 but use the sequencer in the Electribe.
I created a new performance and split the keys into four zones, it doesn't matter what notes. Because I want to use the solo synth of the XW P1 I will not send midi via zone 1. You do this by pressing edit while in the performance screen and choosing midi edit on the second page. Go again to the next page in the menu and turn midi out off. Now the solo synth will play and send no midi data.
For the next 3 zones it's only a little different. Go to the next part by pressing the part + button. Make sure midi out is on and do the same for the next two parts. Now you should have 4 zones, one of which is just the solo synth and the other 3 a mix of sounds from the
XW P1 and the Electribe/other drum machine.
This might be what you want, but you can get rid of the layering in zones 2,3, and 4 by lowering the volume of the respective zone to 0.
Now we haven't left the midi edit menu, but we're going to go to page 1. It has a channel for in and out and gen out for each channel on the XW P1, but we're only dealing with the ext in and ext out for zones 2-4.
So, while still in the menu, use the part selection buttons to go to zone two. I'm going to assign ext out to channel 1. Now when I play keys in zone 2 I will hear channel one of the Electribe. The method is the same for zones 3 and 4, but they will be set to channels 2 and 3 in my case.
Any of the parts on the XW P1 can seemingly be used as a midi controller for all sixteen channels, but this is how you can use the zones to emulate multiple synths at your fingertips.