You don't need a special program, but I suggest Firefox. What you do is
1)Open up two windows, go to runescape on both and go to same world
2)Minimize both so you can see one at the top half, the other the bottom half of screen
3)Type in the login information, but DON'T click login yet.
4)Make sure you can see the login button on both screens.
5)If you have a fast mouse, it shouldn't be hard, but you have to click both really fast, or else it's very hard, as you might have to reenter login information again if it messes. up
6)This works for me 99.9% of the time, and I've done it for over a year, and never been caught ;D