Using a proxy server should do it. Have two rs windows up, enter ur information, place ur cursor on log in button, do the same with the second window, click on the first one, then quickly do ALT+TAB, and click. This should work w/o a proxy server.
Open 2 tabs on firefox to the rs page
Write in all the info
Log one in and fastly log the other one in
might not work on first try, but it will (gotta get timing down)
Yeah, RSbot has a tiny window, but if you're multi-logging, you most likely aren't going to be actually playing both or for very long, so just tough it out with two.
Open two windows and put all your info on them, make one smaller and have your mouse over the bottom screens log in button, then click enter so that the top screen logs in then click your mouse so the bottom screen logs in aswell.