An Entente and Second Internationale alliance against the Reichspakt is much more likely because Germans would be the bogyman of both Entente and the Internationale and which ever side one in an Entente-International conflict would be in mortal danger from the Reichspakt.
Except that an alliance between the Entente and the Second Internationale is almost impossible lore-wise. For the simple reason that they are mirror images of each other, created as a result of huge civil wars/revolutions that happened in their countries.
Asking the Union of Britain and Canada to work together is almost impossible: both considers themselves to be the legitimate government of the British Isles and have no desire to cede their right to the other. Same with the Commune and National France: both consider themselves to be the legitimate government of the mainland and have absolutely no wish to let the other take its place.
The only way for the Entente and Internationale to work together would be if there somehow was a possibility for them to peacefully resolve their conflict. Like having moderates take over on both sides and try to negotiate a compromise with the other side. The issue is that in practice this would probably mean fusing the two factions because I don't see how a compromise between the Internationale and the Entente wouldn't result in a unified government in both France and Britain. For example, if it was possible for the UoB to call back the King BUT he has to acknowledge the legacy of the revolution. In France, it would probably be a Republican Restoration BUT with stronger worker rights.
And even acknowledging that, that would still require an extremely narrow scenario because only the Entente picking its most left-wing options and the Internationale its most-right wing option would make this feasible. Hard to imagine the Action Française collaborate with the Jacobins for example... King Eddie the Fascist working with Mosley would seem possible in an alternate reality, but here the fact they'd basically be competing for the same seat prevents that in Kaiserreich.
Finally, gameplay-wise it's probably a question of balance to keep the Internationale and Entente at war. If it was possible for the two to cooperate, you'd either need Germany to be a bit OP (something which is already an issue) or you would create an OP alliance this way. Think of the reason why the Devs have eventually renounced to the possibility of avoiding the American Civil War: it essentially made the US way too powerful.
I'd also discuss the Reichspakt being a mortal danger for the Entente and Internationale. Both have gripes against it, but it's probably easier for the Reichspakt to pick a side in their conflict and maintain good relations than to annihilate both. The Entente would probably safer because it's basically the old world the Reichspakt is more used to: the Kaiser and the King are cousins, and even if the French are troublesome at least the Nationalist ones are closer to the politics the Reich is used to. Doesn't help also that the Internationale's primary objective at the start seems to aim for a war with Germany, especially in the Commune.