The reason I want to split the keyboard is because I was messing around with the arp and wanted a bass sequence on the the left and an independent lead on the right. Also, this synth reminds me a bit of a Sequential prophet Rev2 and that has this feature and I can't afford one.
What you suggested about using the host to accomplish this seemed a bit of a faff. But I googled "Cubase-split keyboard", and it suggested I use the transformer. Well after messing with that for a short while I lost the will to live and gave up. I then googled "Studio One - split keyboard", and someone in a post suggested to use the "multi-instrument". I had no idea what that was, but ended up experimenting by dragging two instances of invader onto the same track. Success ! A brilliant bit of clever design by Presonus ! Now I can see both synths and quickly set up keyboard splits - and without using two separate tracks.
OK, so that problem is solved, but a new feature request would be a "hold" button on the arp.