I have another account on a separate instance. Is there any way to merge or sync them? Or are they forever separate?
I read that some instances don’t allow posting on certain other instances, etc. I thought I would create an account on a smaller instance, in case this one gets too bogged down or whatever. Is there any way to sync them up?
They're separate accounts on separate servers, so currently there's no way to merge them. It would be like asking if you could merge your Gmail account and your Yahoo email account.
But "forever" is a long time. It's completely doable from a technical point of view to merge two Lemmy accounts (and doesn't require two different companies to work together like in the email example), so maybe that will be a feature at some point. Because of the potential for instances to change policies or defederate with others that you care about, many people have asked for a way to migrate an account from one instance to another (copying all the post/comment history, subscriptions, etc.), which is a similar problem and one I bet we'll see sooner than later.
It's not necessary, for sure (I personally only have this account), but reasons I've seen are:
In case your home instance is down, you can still browse and post
If you're interested in content in a community that's on an instance that your home instance isn't federated with (e.g., you're on lemmy.world and want to interact with a community on beehaw.org)
To keep some kinds of activity separate from others (e.g., some people have a separate account for porn, usually on lemmynsfw.com)