The turnstiles for the north lower are at the North west corner for st johnstone fans, same with the exit gates, the turnstiles and exit gates for the north upper are all away along the north stand concourse ( Janefield Street ), could easily put a temporary fence at the north west corner.
The only question is will they let the United fans in the North Upper above st johnstone fans? they let Celtic fans sit in the main stand upper when Hibs handed back a few hundred tickets, they did leave the first few rows empty though.