I have run into situations where having both FSUIPC and FSX trying to control axises will cause fluttering of the control surfaces, as FSX and FSUIPC fight each other over the controls. If I recall correctly, Peter states in the FSUIPC manual that if you use FSUIPC to handle the controls, turn off (disable) the FSX controller
It sounds like Linda is your thing then, but yeah, if you dual map
controls then obviously you are going to run into problems, but nobody suggested doing that. A lot of people map their primary controls surfaces straight through FSX, since those are common for all planes, map their brakes through FSUIPC, map common button controls through FSX, buttons that must be individualized through FSUIPC, and then use something like the A2A Configurator when you need something like a plane-specific turbo lever. No conflicts or fluttering at all. But I guess that is the beauty of the system, lots of flexibility there, and each can do it their own way.