I suppose this will happen if a club has a Ist team and a "ressies" squad.
Have a look at the SCEFL and there at least ten teams that have reserve teams playing in the KCFL. e.g. Lydd Town,Lordswood,Rusthall, and Snodland. Even the Rams have got a team in the KCFL!

The whole point surely of a reserve team is to keep any first teamers fit(returning from either injury,suspension or just if they're not getting "game time" for the first team)) and or to progress any young guns to see if they're good enough for the firsts!
No -ones going to moan if a reserve player leaves the "stiffs" to play for the firsts,are they?
This situation has been going on since teams were lucky enough to field more than one side on the day. Just got to live with it.
I think it could be called "Squad Rotation"