I'm reading the SNCB conditions of carriage to try and find a definite answer to the split ticketing thing
If you want to help it'll be here somewhere
https://www.belgiantrain.be/en/support/terms-and-conditions-for-transport
I haven't found anything yet but I thought this was a nice detail on the Weekend Ticket:
"Si votre gare* de départ et/ou de destination fait partie d’une Zone*, vous pouvez circuler librement toute la journée à l’intérieur de la ou des Zone(s)*, sans en sortir."
Roughly... "if your start of end destination forms part of a zone (e.g. Zone Brussels, etc), you can freely circulation all day inside the zone, without leaving it". Quite useful, essentially giving you a free all-day S-Bahn ticket within Brussels
This is also VERY useful for people wanting to use the coast tram: "Si la gare de destination de votre billet aller-retour est une gare du littoral, vous pouvez effectuer le trajet retour de n’importe quelle gare du littoral. Ces gares sont : De Panne, Koksijde, Oostende, Blankenberge, Veurne, Heist, Duinbergen, Knokke, Zeebrugge- Strand/Dorp."
Basically, if you're getting a train to the coast on a return ticket, your return ticket is valid back from any station on the coast! Presumably to encourage people to use the coast tram.
Hopefully I'll find what I'm looking for now re: split tickets