Hello all, I'd like to add some observations I have made in regard to this. I believe the underlying issue is when mouse control passes from the primary to the secondary PC, the Everquest client on the main PC is still focused on the primary PC. The reason that X-Controls work for some people (those who move the mouse to the screen edge) is because that program removes the focus from the Everquest client when the mouse leaves the window, and prevents the shadow controls from coming through.
For myself, I use hotkeys to pass control to my secondary PC and my mouse stays over the Everquest client. So my client never loses focus and the mouse look on my secondary PC is shadowed to my primary. I can replicate the behavior by simply clicking out of my main PC Everquest client (so it loses focus) and then passing control to my secondary PC. When I do that, it works as expected.
I don't know if this is a feasible suggestion or not, but if there were a setting in Multiplicity to defocus the active window (or a specific window name) when control is passed to a secondary PC, I believe the issue would be resolved.