I think its always better for game pacing if you split the Axis team (Germany_Italy and Japan) instead of splitting the Allied team.
The reason is basically that the Allies require more coordination, which results in more pauses in the action so that they can discuss strategy, which in turn causes the pace to drag. If you have 4 players then this isn’t an issue, but if you only have 3, I think its better to just have a single Supreme Allied commander. Japan and Germany_Italy are set up such that they only need a very minimal amount of coordination, and even then only durring the endgame. The Allies by contrast, need to have a plan worked out from the get go.
In previous versions of the game, a split Allied Team also made the need for an Axis bid a little less critical, because invariable someone on the Allied side goofs a key coordinating move somewhere along the way.