I'm pretty sure it was the 18;47 from Salisbury and 20;50 from Waterloo which was the booked 2 coach working. Previous timetables had that unit stay at Waterloo to attach to the 159 from Exeter to form the 21;20 service.
Perhaps the wrong time frame but the Dec 2011 CWN, which I have, shows the 1950 (ex 1747 Salisbury) as a single 159. The 1847 does what you suggest, forms the RP of the 2120 (also a 159).
However, the February 2007 CWN shows the 1950 as a single 170. This seems to be the only 170 at Waterloo in that particular timetable.
Going back still further, the March 2004 CWN shows four single 170s off-peak SX, at 1003, 1103, 1403 and 1503. These are all Salisbury workings, except the 1503 which extends to Gillingham. There are a good few other 170 workings at Waterloo, attached to 159s.
Bizarrely the March 2004 CWN shows "2 159" for some workings (in combination with a 3-car 159). Never heard of a 2-car 159 so I presume these were actually 158s? I'm not sure whether the 158s and 170s co-existed on SWT, I have to admit.