I think there
is a path for this to happen pre-1969, along the lines of what
@The Lethargic Lett suggested, and what I went with in my list, but it requires a very specific series of events starting in the 30s and 40s. It requires the Tories to fully embrace, rather than merely flirt with, social democracy, for the Liberals to stubbornly stay true to their classical liberal direction, and for them to
stay on these paths. It's the latter point that I think is hardest; even though Tory socialism and classical liberalism are real ideologies, the fact remains that the
parties have their own histories and traditions; the Conservatives represented established power structures, while the Liberals had a reformist streak to them, and it's going to be tough to have them each do a 180; I mean, it's those exact histories that lead to the LPC bending towards the reformist left while the Tories ultimately marginalized their Red wing in the 80s.
In my list (I swear I'm not trying to shamelessly plug myself here), I started with the twin ideas of 1) WLMK winning in 1930, and taking a very cautious approach to the Great Depression and 2) H. H. Stevens (who IOTL bolted from the Tories to start his own economicaly-intervenionist Reconstruction Party) challenging and winning the Conservative leadership from Bennett (as almost happened IOTL), then taking it down the economically-interventionist route he preferred. Other things you'd probably need to do is a more thorough embrace of the farmers' movement (but
not Bracken, who was small-c conservative and Liberal-aligned), and marginalizing figures who used "red scare" rhetoric (such as George Drew). On the CCF side, a stronger/longer lasting influence of the social gospel would likely help, too. Then… after a few decades of this, and some electoral pragmatism (a dominant Liberal Party?)… you can get your merger. But only then.