Cool, a 2 into 1 carb thread. I'm in the middle of doing this myself having been inspired by the guys on the Kawi forums who all seem pretty keen on the idea. I'm not after any performance changes, just doing it for fun really, lol. Picked up a 34mm Keihin PWK for when I get the manifold sorted.
A lot of the Kawi guys in the USA seem to make the manifolds out of large diameter copper pipe from plumbing suppliers and braze the joints, which is a pretty cool idea.
There is also a bloke on the TCU forum called Choppersprocket who makes them I think for the XS400. I also found sombody here in the UK who makes a cast 2-1 manifold for the XS650 and asked him if he would consider making one for the 400 but he said no
I was thinking of trying to make it out of steel exhaust bends done with a mandrell but I'm still having a bit of a think about it
Sorry HT you answered the question already...