Have no problem with it if they can behave well enough.
I'm sure many of us have been in the home end at away games for one reason or another.
I did so at Berwick one time and have done so at many Scotland games.
We filled Kilmarnock away end at one game and many fans were in their end.
Anyway, back to the main point, which imo, is to maximise revenue at every game without causing any danger to either set of fans.
If a few Celtic fans want to sit in with the Thistle fans to watch the game live, I don't see any problem.