Pretty sure combining communities isn’t possible, from what I’ve read the idea is that after a bit the most popular incarnations of any particular community will win out and the problem should somewhat solve itself. Don’t think that solves the problem of being overly reliant on a single instance to host that community tho.

There are some duplicate communities (e.g. cats@instance1 and cats@instance2 to use your example) but you can subscribe to and interact with either/both communities without leaving your chosen instance - you don’t need to make multiple accounts.