okay,a open 2 internet browsers, they can be the same kind, it doesn't matter. you can also use the runescape client window, or a bot client.
anyway, log both accounts in and select your world and everything until you get to where you push the play now button. the line up your windows so both buttons are directly on top of each other.
then, click play now on the top screen, ALT+TAB to the other screen, and push play now.
if you do it fast enough, you will log into both accounts, in the same world.
its easy really