A little heat from a soldering gun tip to both ends of the screw can help. People put all kinds of stuff on the screws when they assemble guns.
Schellac, nail polish, linseed oil, varnish, super glue.
They all work in various capacitys as a thread locker.
Super Glue will freeze it up so tight you'd think it was done with green LockTite, but you have to assemble the parts FAST!
Anyway..keep the heat on the screw itself. It'll transfer to the escutcheon(s). You don't want to heat the wood any more than the process will do on it's own.
One reason they may be locked up in the first place is that one or both of the grip escutcheons was loose, especially the threaded one.
They couldn't tighten the grips in place so some locking formula of whatever type was squeezed into place around the escutcheon to freeze it in place.
SuperGlue works fast like I said in the thin voids that allow the ecutcheon to slip and hold it in place fast. Any liquid stuff still dripping around that gets into the threads when the screw is run thru will do the same to that screw when tightened.
A soldering gun will loosen the stuff, but if the above assumption of the loose escutcheon is also correct, that'll start turning again with the screw. You'll have to at that point hold the escutcheon in place with a scribe point or other small tool while trying to unwind the screw.
3 hands helps or just your two and a bench vise to hold the pistol.
They can be a real pain sometimes,,other times they back right out.