On the rare occasion(s) the 47 was used, it was to provide additional traction on the Inverness route where there is a Load 8 and a few steep climbs and hence pushing it to rely on a single 73/9. Hence, 2x 73/9s are normally used on this route, however when there's been a shortage of 73s, a 66, 67 or 47 has piloted.
Both locos are under power, working in tandem (not multi) - the 73/9 hadn't failed. The 73/9 provides traction and ETS, the 47 just provides traction. In this arrangement, both locos need a driver.
As noted above, when a 66 (or 67) is used to pilot a 73/9 on a similar basis it can multi via the AAR system with the 73/9, so the two locos can work in 'proper' multi and only require one driver.