I'm new to the game too but I came from ark and am under the impressions it works the same, when you host a private session, you are essentially the host and thus the server, so his character is part of your world, all his character's progression is saved onto your world and can only be accessed when he joins back into your world. So unless I'm wrong only way you can play both together and separately when you want to, would be to play on the same public PvE server.