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.