Belgium is clearly divided into Flanders and Wallonia. However you hardly ever see them separating into independent states in ATLs. Assuming POD no earlier than the Treaty of Versailles and Belgium neither conquered nor puppetized how plausible is it to split into Flanders and Wallonia?
Unlikely, even today in Belgium 80% of the people doesn't want a dissolution and before WWI there was no separatism, only a Flemish Movement that fought for the equality of Dutch and French.
During the war a separatist movement formed by frustrated young marginal politicians, the 'Activists', collaborated and was hated by the entire population (example: the Flemish Movement had a petition to change the French speaking university of Ghent into a Flemish one with 150 000 signatures, the Activists had to make a new petition for the same goal, which only got 5 000 signatures)
The Activists ruined the situation for the Flemish Nationalists after the war, which lead to a radicalization of a fraction, which resulted in collaboration during WWII, they weren't known as the Activists then but as the 'Blacks'.
After the war the people and certainly the resistance demanded a harsh repression and because the Belgian government had no armed police force they had to let them do, but managed to judge every case in or civil or a military court, which slowed things down a bit.
But the blacks were exiled in their own country and formed anti-Belgian groups, that wanted to destroy Belgium and it's institutions, today these people formed parties such as Vlaams Belang and NVA. The first party is slowly dying and NVA is only popular because of Bart de Wever, but the majority of the voters still voted on the traditional parties.
Even now the end of Belgium will be unlikely because Flanders has everything to lose and nothing to win. It has been like that ever since Belgium came into existence as an independent state.
Plus the fact that Flemish and Walloon already form one nation for 5 centuries already and share more culture and mentality then most people think.
PS: what also helps is that there is no ethnic devision in Belgium, but merely a linguistic one.
And the Flemish regional day (11th of July) celebrates a rather curious battle, cities from Brabant and the kingdom of France fought vs a few cities of Flanders and Namur, so it's not a Flemish victory at all.