It's possible.

Was a joke.

No, the other rationalism, rationality. My bad.

Are you sure? I've seen posts speaking of 'aspiring rationalists'. It does make sense that rationalists would see themselves as rational, but it does not make sense for rational people to call themselves rationalists. Rationalism is sort of like a belief in power of rationality. It's to rationality as communism is to community.

Believing that the alternate realities must exist if they are a part of a theory, even if the same theory says that the worlds are unreachable, that's rationalism. Speaking of which, even a slightest non-linearity is incompatible with many worlds.