The host(s) is/are the ones that are the most our or function in daily life, so if you have more than 1 host, yes they can visit this other host too. You're all part of each other so in theory everyone can meet everyone else
That would depend on how you inner world works. For instance, no one can come into my part of the inner world except me and Leo, so no one besides her can visit me. Where Cassie & Jacob live though, they are visited by several other alters

There's no reason why hosts and the other alters (both are alters, of course) shouldn't be able to meet, at least in theory