There's no such thing as "Lemmy together", Lemmy servers are individual instances of "Lemmy software" created and managed by different individuals that are totally independent from each other.

It's like individual subreddits, managed by different mods, each one with it's own rules and beliefs.

Weren't some subreddits split and hated each other? YES

Did that make reddit not valuable as a platform? NO because there were still many subs with amazing people and quality content.

When you have millions of people, divisions are inevitable, it will surely happen here if Lemmy gets enough traction, but I don't see it as a problem, reddit was fine regardless of it, Lemmy will be fine as well.

"Who calls the shots when everyone has equal say?" this is pretty much one of the core design goals of federated services. You get say in your server. People will federate or defederate naturally between servers, that's... The point.

it won’t take long before the community is split on one.

People are always "split" on all kinds of topics.. Conflicts and disagreements are part of the human experience and impossible to avoid.. The question is how you deal with conflicts.. Trying to avoid or suppress them doesn't work..

Who calls the shots when everyone has equal say?

Nobody calls the shots, that's the entire point.. People are free to form communities and run their communities however they see fit.. If a community has an issue with another community in significant enough ways, they will block each other and that's it..

And it's not like this is anything knew.. The internet is inherently diverse when it comes to different opinions, and that's ok..