Not off the top of my head but braided hose and fresh fluid makes a big difference, and when (some time ago) I combined that with a new disc and pads and a caliper rebuild the front brake was great, certainly they were good enough to overpower the front tyre...
When you say they're poor what do you mean? Lack of bite, lack of power, lack of feel, fading?
I would imagine there is a way to twin it but personally I'm not convinced it's worth the aggro, there are a number of XBR outfits running around with single discs, the clutch however... doesn't like big loads and start stop riding!
