I suspect there is less to this than meets the eye, despite it having its origins around 1850, when the Farnham branch was built. The earliest map I can find in NLS is 1870s, so well after that initial enlargement, but before the building of the New Guildford Railway via Cobham and consequent rebuilding to roughly what is there now.I think that one factor in 6/7 being two-sided is lack of space to fit two lines in without shortening the platforms or extending them inconveniently northwards. The lines came out of the tunnel, past where the shed was and can only then spread out as they pass under the Farnham Road bridge. If there was then a further point to lead into double-track, the platform ends would be even further away. It is all a matter of compromise.
I also suspect that if you asked a Victorian railwayman about having one track/two platforms - is it a good or bad thing? - you'd get a blank look as it was not regarded as a thing at all. It's just what happens if you add an island platform to a station with side platforms.
I have some information about the RGRR (taken over by SER as soon as it was built) but almost none about the LSWR. So I'm rather guessing that the first station had two side platforms (2 and 3 - I'm using today's numbers) with buildings along the outside of P2. The RGRR relied on LSWR to build lines to Ash and Shalford, and had to twist their arms to get this done quickly. I guess LSWR had other priorities, but still would much rather build and thus own all the lines through their station. Does that mean what was built was entirely to LSWR's use, or did RGRR/SER have any input?
Assuming the 1871 map shows what was built in 1850, an island platform was built for the RGRR/Farnham traffic to use. This is a very large scale map - 1:500 - and the surveyors have included very detailed representations of the trackwork. However, trying to work out how the platform lines were used is probably trusting that detail too far, and indeed it does not make much sense if you try it. It does however look as if the track pairs from Ash and through Chalk Tunnel both merge to one line which then divides around the island.
In 1885 a further pair of tracks had to be accommodated for the Cobham line. This was led into what had been the main lines (P2&3), and those main lines were realigned to roughly where that P3&4 line had been. A new platform P5&6 was built, and a new (rectangular) island P7&8. Clearly at this stage the one track/two platforms feature was not worth eliminating then, presumably its advantages outweighed any drawbacks. As to what those advantages might be, we are all still guessing.
