Having ordered a KVM switch. I played a bit more and got things to work.
The steps.
1) Uninstall multiplicity (on both machines) and re-install from latest download.
2) Change home network to Private rather than public
3) Enable MeshNet in the NordVPN software (on both machines)
4) Delete secondary from seamless and connect in KVM mode.
Hurrah - it works - I had two mouse trails with the secondary mouse pointer offset by some distance from the actual mouse pointer. Changing the resolution of the secondary display to 1080p seems to have fixed that too!
Thanks to basj and DrJBHL for the tips...