You can just multilog. Download multiple clients from the website, and then load them up at the same time. Then, after typing in your log in info for both accounts and going to the lobby on both accounts, pick the world you want both accounts to be in. Then, very quickly, hit play now on both clients, and then boom, both accounts are logged in. You can do this for as many accounts as you like, but the key is to hit play now on every client as close together as possible. If an account is already logged into the game, this method will not work; every account needs to be in the lobby to start.