Well that’s kinda the point of my quick suggestion in the original post.
Instead of committing to federation, how about committing to aggregating clients that allow you to do exactly this. Right now, there’s no app that will work for both lemmy/kbin and mastodon/microblogging. No way to unify the notifications or even combine the feeds or just have a unified interface for the two platforms (that are, let’s face, both just full of text messages and feeds).
By allowing each platform to be distinct but remain open with their APIs and “play nice with each other” while leaning into the value of aggregators as a primary part of the value proposition of the system, users might be better served.