Everything will depend on how currently allied Powers will behave between each other.
If these Powers will be able to undermine each other and struggle for dominion over the same systems (even if in a more political way), the natural consequence is that those Powerplay communities are going inevitably to split, because they would become actually enemies, or at least "competitors".
Right now there's basically three major communities (Alliance, Empire, Federation) and 4 smaller ones (Kumo, Syrius, Utopia and Grom), I expect this situation to change in 12 independent communities, one for each Power.
The decentralization that seems will be a major characteristic of Powerplay 2 will make difficult (if even impossible) to maintain reliable diplomatic relationships between the different Powers, the only kind of diplomatic relationships will be between the communities supporting each Power and they will probably never represent a Power as a whole.
That's assuming that Allied Powers will be able to attack each other of course, which I believe (and hope) they will be able to, because a consequence might be people pledging other Powers to free systems for the Power of their own, and that would be a silly thing gameplay wise.