Isn't this patched now?, use to use this method. doesn't work for me now.
Can't you just change some line of code in one of the applet files?
In my opinion, the best way to do multi logging is just simply opening up two windows of RS, fill in the account information and click 'Login' on both windows as quickly as possible. I use this method in RS07 to craft nature runes with 5 accounts at once =)
just open rs at the same world on 2 windows and use alt tab to switch between screens... you don't have to be super fast and it'll work.