I have built many wheelie bars and rode a few also. I will give ya a little insite..
The single upper bar is fine and they work great but, When you use two smaller uppers it actually is a smoother transition on and off the bar. meaning when the wheel touches the ground the two smaller bars flex more and makes the the trasition a little less noticable. Also it seems like the the two upper bars gives you more stability side to side. I know these are trivial but beleive me you will notice the difference.
on my bike I would not put the tube between the upper and lower tubes. you get more flex that way and it is much more comfortable. feels like a "lazy boy", but you need to watch that on single upper style bars.. the bar can twist easier if you get side ways.
You will need to tie the top bars together with a tube and tie the lower bars together with a tube. this will stop the bar from twisting.
The single upper bar is always easier to build and maybe a bit lighter, but i think if i was going to build any more i would only do the twin uppers.
ATV drag Wheelie Bar **NOTES**
Always use heim joints to attach wheelie bars, Never rigid mount to chassis
Never allow people to grab and pick the bike up with the wheelie bar, unless they are at the very back near the wheel.
Always check the bearings in the wheel before every race weekend.
Check welds for cracks before every race weekend.
try to use continueus bars for upper and lower tubes with min welds, if you must weld cros bars extra, never weld completely around the bar.
Your wheelie bar legnth should be a min of your wheelbase.
NEVER use mild steel or conduet for your wheelie bar!
attachment points on bike should be your rear carrier since bolts and a point high on the subframe.
If you are running a shock, NEVER put a brace between the upper and lower bars.
tubing sizes, 4130 Chromoly
twin uppers use 3/4" dia .065 lowers 3/4" dia .049
single upper use 1" dia .065 lowers 3/4" dia .049
Wheel should be light but as wide as you can get. You must use a bearing on your wheel, No bushings.