Dual filter heads do seem to be thin on the ground. To be honest Knighty, I think the way it's done now is to put two singles together, which is why the single filter heads have four ports. It makes sense really, only one casting to make.
I had a look at the one I got, and it does seem possible to join them up either so that the fuel goes through one filter, then through the next, or, so the fuel is divided equally between the two filters, if you see what i mean.