My personal opinion is it doesn't matter. Some of the very best engines across our dyno (our own engines and from other big name builders) have been single bearing. Mind you, it's not the bearing configuration that made them any better than another.
Consider this...A ball bearing is not designed to carry a lateral load. It is designed to carry / roll free when loads are placed vertically upon it. Unfortunately, when you enter a turn with your go-kart, the weight of that 6+ pound flywheel pulls the crank laterally due to centrifugal force and now that extra flywheel side ball bearing is expected to carry that lateral load. In reality, it can't do that as well as a bushing. In fact, if you pull the flywheel to the right enough to take up the crank endplay while you're dynoing the engine (by pushing on the pto side coupler and crank) you'll lose rpm due to the drag caused by the ball bearing being loaded in a way that it was never designed to handle. This is the reason that many years ago WKA had to address and make a rule to prohibited radial bearings and the likes on cranks. Now, on a limited, or open, you see guys preferring the single bearing blocks because of the issues of breaking the case under the flywheel side ball bearing where the oil drainback area is. With stockers pushing 7200 today, we're not all that far away from limited and open rpms of a few years ago. Could this "rattling" effect be causing other problems (ring flutter, ring seating, etc) with our flatheads today? Something to consider at least.
Look at it this way... If ball bearings were so great, we'd have them in the front hubs of all of our every day drivers (pick-up trucks, etc.) No, we have taper bearings in almost all cars on the road today -- Why? Because they can handle the side loads placed on them so much better than a ball bearing ever could.
With that said, all of the new R4 flathead blocks available are dual bearing because that's what the factory felt the majority of kart engine builders wanted (and that's probably correct.) For years, you saw engine builders list their pricing as a "standard" shelf engine with single bearing, and their "national level" engine with dual bearing. After awhile it was pretty much expected by the racers.
If you're looking at used flatheads, don't get caught up in single or dual bearing arguments -- be more concerned with overall performance of the engine. I look primarily at the intake port size, volume, shape, and seat alignment. Almost all of my own kids' engine are single bearing -- if I felt it was that important to have dual bearing blocks, you bet I'd have them for my kids.
Thanks and God bless,
Carlson Racing Engines