Sure, each one's developers may prefer that they get used instead of the other, but being federated systems. But there's no reason not to have transports or gateways between them. They're documented protocols, both communities are committed to openness so we won't have any of the nonsense where AOL went around IP-blocking any well-used transport. A user gained by either is a user both can talk to, at least potentially.
The Vulpine Club is a friendly and welcoming community of foxes and their associates, friends, and fans! =^^=