Trains from Southampton, via either Chandlers Ford or Redbridge, always terminate in the Salisbury bound platform (down Salisbury - platform 2), and there are signalled moves for reversing back south towards both routes, via a crossover immediately south of the platform, I don't think there is a signalled move from Chandlers Ford into the southbound platform (up Salisbury - platform 1). Trains arriving from Chandlers Ford use normal LH running over the final two track length, (about ^ mile I think), into Romsey, and as far as the drawings in the online sectional appendix are concerned they are on the 'main route', the Redbridge direction is drawn as the branch, surprisingly.
Here's a Saturday of reversals at Romsey, showing all passenger services arriving in, and departing from, P2:
http://www.realtimetrains.co.uk/search/advanced/ROM/2015/09/12/0600-2000?stp=WVS&show=all&order=wttThe track layout requires that a train from Salisbury must enter the southbound platform, and there is another trailing crossover at the north end of the station to allow it to return to Salisbury, I don't know for sure if that is a passenger signalled route, but there seems to be no other practical way of reversing directly towards Salisbury, there are no facing crossovers on the line in that area.
I have had a dig around in real time trains to see if I can confirm what happens with a south to north reversal. Here we are, in late September
FGW▸ are running via Eastleigh and Chandlers Ford during a Southampton closure, and
SWT▸ are running a Salisbury to Romsey service. The latter arrive in P1, and shunt to P2 via a particular signal:
http://www.realtimetrains.co.uk/train/Q88041/2015/09/27/advancedUnfortunately I can't confirm the whereabouts of that signal, but give me a few days and I can...
Hope that helps.
Paul