In terms of Fort William AND Oban, splitting at Crianlarich would be a bit messy but joining would be an operational nightmare at a location without conventional signals and where the loop points are, to quote the Sectional Appendix ,'hydraulic spring-controlled points - splitting and joining sprinters is pretty straightforward, doing it with loco hauled trains is not, unless the train ran in each direction with the loco for which ever was the rear portion in the middle of the formation and it was then a simple sprinter style split/join.
It would be possible, if a little convoluted, for two Up-direction (southbound) loco-hauled trains to join at Crianlarich without having to manually operate the hydro-pneumatic points at either end of the loop. Here's how:
1. The first train arrives in the Up platform and gives up its token.
2. The first train obtains a shunt token. The loco uncouples and runs round its train via the Down platform and couples onto the rear of the coaches.
3. The first train propels south onto the single line until clear of the points. It draws forward into the Down platform and returns the shunt token.
4. The second train arrives in the Up platform and gives up its token.
5. The second train obtains a shunt token and runs forwards onto the single line until clear of the points. It propels into the Down platform, and the coaches from both trains are coupled together. The second loco returns the shunt token.
6. The first loco obtains the shunt token and uncouples from the coaches of the now combined train. It runs round the train via the Up platform and couples onto the second loco. The shunt token is returned.
7. There is now a complete train sitting at the Down platform with two locos at the south end. A section token is issued and off it goes.
The above is only intended to show that it can be done. Alternatively, the joining procedure can be carried out without any running round by resorting to one manual operation of the points at the south end.
Instead of double-heading, an alternative would be to have one of the locos wait overnight at Crianlarich and take a portion of the northbound sleeper.