I think it is unlikely to be solely a carburettor problem, what is the likelihood of two carburettors, both operating at different heights developing a simultaneous fault at 20 miles into a run, you would at least expect a period of time when the bike would go onto one cylinder for a while, same as when you run out of fuel, I would definitely drop in a new condenser and maybe even a coil, but try the condenser first.
A rider at the Austrian leg of the recent International Rally had a breakdown on the road, when I arrived on the scene he had the situation under control with carbs off the bike, slides out, float bowl covers off, fuel all over the place, so convinced was he that the problem was fuel related, I could not do much to help, but as a last resort I offered to pick the bike up later in my van, but later on I bumped into him at the hotel, when I asked him how he eventually solved the problem he said he thought that the points heel was running a little dry on the distributor cam and the points gap had closed up to the point where the ignition would not function, such a simple fundamental problem, but so easy to overlook or ignore in the heat of the moment, and jump to the wrong conclusion, I am sure we have all acted similarly at some time or other.