The warp is unpredictable. Why not just say that one of the ships came out of the warp where it shouldn't? Which turns out to be, near the other planet/fleet.
And there is no need for betraying each other
PS. Or like with Space Marine chapters. The original cult from one planet divided and one of them went to the other side of the galaxy and stablish it's own cult. And they keep similar look to the original