Instances with public domain names could successfully federate with onion instances, as long as it's capable of accessing both networks.

Onion instance talks to the public instance through an exit node, public instance can reach back to the onion instances through its hidden service.

But that doesn't remove the requirement that both ends needs to be on the Tor network, but at least one end doesn't have to have an onion address. And obviously, the onion instance will have limited ability to interact with non-Tor-enabled instances, and one of them being public renders the usage of Tor somewhat moot.

so it would create a second whole feddiverse

I wasn't clear enough in my original post. I was mostly referring to the idea of creating a sort of "Hidden Fediverse" which would be separate from the regular clearnet fediverse.