I just reinstalled the KVM Pro trial version on both PCs and snooped around, but I couldn't find any setting that allows me to do what I want.
To clarify: my main PC has 3 monitors, my secondary PC has a single monitor. When I connect to my secondary PC, I want its display to make use of ALL my monitors on my main PC. That is something I routinely do with RDP, by ticking the Use all my monitors for the remote session option in the Display tab.
From what I understand in the Multiplicity documentation, you might be able do that if your secondary PC has several monitors connected to it, but that's not the case here.
So unless I am missing something obvious, it would appear that this functionality has yet to be implemented in Multiplicity.