I'm going to go ahead and guess that it's your CPU that's causing your freezing, not your memory . One thing to try is shutting down every non-essential service and program that is running in order to free up as much as possible. Make sure your graphics are cranked down as low as they can go on both characters and see if that helps. If so then you can slowly turn them back up on your main.
Still, dual-boxing on a single core is pushing it and since it's a laptop your only solution really is to upgrade to a new one or buy a desktop.