Aye, you make a good point, however both stands hold less than 2000 and not every fan would make the move. We'd probably be looking at around 1500 or so tops. (Away fans, if there were any in the NS, wouldn't be allowed to move). It would have to be around the East side for reasons that LtL points out so a path would need to be laid to connect the stands. I don't see any problem in letting folk move maybe 5 minutes either side of half time either.
It probably wouldn't work if there was any significant number of away fans anyway so, after promotion , could only happen on a limited basis.
I'm all for LtL's idea re any new terrace though, surely the priority in designing it would be linking the two areas?