Just having a dual head display just wasn’t enough. I went the whole hog, bought myself a PCI graphics cards and another identical monitor!
Its all up and running, but I just can’t get 3d acceleration to work on it properly. I’ve only had a quick go, and everything seems to be right, but glxgears just crashes. Its only really a problem if I want to play games, and for the time being I’ve got too many other things to do!