I wonder if you’d be able to use paths to achieve this? IDK how AP would handle it on its end, so I suppose it’s a case of https://tryitands.ee/

Matrix (or even easier XMPP) can coexist on the same domain with Lemmy or Mastodon, but with two ActivityPub services that is AFAIK not possible.

You are able to host all these services and more on the same domain. I do a similar thing with different services at different subdomains. I don’t believe the username@ portion can help point to different services, that syntax is usually associated with email addresses.

I think the best way to go about it would be having different subdomains such as lemmy.domain.tld, matrix.domain.tld, etc.
To accomplish that, I have a wildcard subdomain point to my server, my reverse proxy (Caddy) handles figuring out which subdomain maps to which service on top of handling TLS certificates for me.