it depends on your system ram. In my case, I have only 1gb, so I'll manual set each prime95 instance to 400 each, total 800mb out of 1gb.
in your case you could set them to 750mb each, which gives you a total of 1.5gb roughly. I don't remember but I saw it some where that if you ran more than 750mb each instance of prime95, you will get some kind of error.
another different option would be to get Orthos, it automatically runs in dual core mode for you.