You can call an FP from 2 different PCs at the same time with no problems. You will need to determine which PC is the master so that the other PC will avoid setting outputs on the FP when it's not a master. You can place some code on the FP which will decide which PC is the master and inform the PCs.
The real trick in such a system is making sure that both PCs are synchronized so that when the master PC drops the slave can take over flawlessly. Another problem is what to do if you want to use the slave PC as a UI access point to your system. Since you have the same program running on both sides (so you can have your hot backup) you have to figure out ways to do things differently when the PC is not the master - it needs to send requests to the master which can then decide what to do. You also need to decide what to do with data which is missing because the computer was disconnected.
I have written such a system employing several PCs and FPs and designed to be extensible and I can tell you this is not a simple thing if you want full functionality in your slave PC and if you want to make sure you don't miss anything. What you might want to do is use a single PC as the UI PC and have both PCs log the data all the time and combine it later.