I had an idea where the Progressives or CCF supplant the Liberals in the 1920s or 1930s, and overtime the Liberals are only able to elect MPs from Quebec.
During WWII, with the Conservatives likely in power, a worse conscription crisis leads to the Bloc populaire doing far better than OTL, and the Conservatives losing all their seats in Quebec. With the nation at war, and the government needing representation from Quebec, the Conservatives form a coalition with the third-place Liberals. After several elections with the Conservatives failing to win a single seat in Quebec, and the Liberals failing to win a single seat in the Rest of Canada, the two parties agree to form a permanent coalition, with the PCs not running any candidates in Quebec and the Liberals not running any candidates in the rest of Canada.
Anyway, that's just an idea that was mulling around in my head for a while.