From your earlier tests on the floats, I'd say the float assemblies are sufficiently mis-matched to allow this to happen. You'll likely find when driving, they'll both work but at lower and idle speeds (as you've found), one will likely dry up - as you've found. To save yourself faffing about, I would suggest replacing both floats, then with the new needle and seats you've fitted, they'll be quite closely matched. Noted from your last post, they have been 'serviced' however, with respect here, just from the issues here in the float assemblies, I feel it may have been done a bit hap-hazardly.
Also, while you haven't raised it as an a issue here, what's the condition of the butterflies, shafts and the bushes in the bodies ? With twins to get them set up and to work as best as they can together, as Nick mentioned, the characteristics of them need to be as closely matched as possible, and certainly, these parts have a bearing on this at lower and idle speeds. The 'Drop Tests' (of the Carb Pistons) that Nick also referred to is also important and again, need to be close to each other. The Pistons and Bells are actually matched to each other when manufactured, this is something that's not always appreciated and inevitably over the years, especially on twins, they can get mixed up, so when doing the tests, if you find them not closely matched, swap the Pistons between them and try again, but just be sure there's no binding or interference.
When fitting up to the Manifold, it's important that the Throttle Linkage in particular has some end float, while I don't measure it, I'd say I look for at least about 0.020" (0.5 mm). If this linkage it tight, you'll find you can get them to idle properly and usually 'hang' at higher idle revs.
This hasn't covered everything you need to check, it's all attention to detail, but this should get you going with them.
