Usually users can post from any instance to any federated community on any server.
Unless the federation between lemmy.ca and
Lemmy.ml is wonky/broken, one of the servers is overloaded, or they’ve explicitly forbidden it, you should be able to post there.
Iv tried but I think I need to set up a specific account for that lemmy instance in order to post.
eventually some sort of single sign on will probably be developed, will try to give it a go myself
Usually users can post from any instance to any federated community on any server.
Unless the federation between lemmy.ca and Lemmy.ml is wonky/broken, one of the servers is overloaded, or they’ve explicitly forbidden it, you should be able to post there.