I flown several kites that were rigged for dual or quad. The double strand is line of bridle line, folded in half. Now there are two strands (or legs). If you use High Test bridle line (100#) it is very easy to tie and un-tie,... so you can make adjustments easily and still keep as few knots as possible when testing/tuning. You are placing a single overhand knot in one leg only. When you larkshead to this pig-tail you will grab both strands, even though only one has a knot.
At the top spreader location (you are not removing the existing dual line bridle at all) you will slip that folded/doubled bridle line thru the hole in the sail and wrap the spreader fitting such that nothing can move when you yank or spank, pull or jerk the line. This become the top flying lines attachment point when you switch to flying the kite quad-line. The attachment point is still the dual line bridle for the quad lines on the bottoms.
I've flown TC Ultras, ordinary 8' sport kites and the Prism 3D using this method,... but WAIT, there's more.....
If you order today you can get a 2nd one free (you just have to pay add'l shipping and handling! < just kidding > )
Charles Stonestreet is an older gentlemen with typical age-physical symptoms. He can't grip a quad handle anymore! So he flies a THREE string method (It's still considered mutli-line). He flies the kite on the normal two string bridle using a wooden handle (imagine a very shortened broomstick, rounded off on the ends and drilled thru at each corner for affixing the flying line leaders. He wrist rotates this stick back and forth is his grasp. THe braking action comes from that third line,.... tied to the spine (center fitting) this time, so that by changing the relationship distance between the handle and his waist he can add forward drive or take it away. Oh, I almost forgot, "Stoney" flies one in each hand at the same time. A great show if you haven't seen him in Freestyle (OIOU) !!!
So, how long are these new top line attachment points (or the spine thingy is your going after Stoney)? You'll have to experiment and I highly recommend you undertake this mission with a partner. Testing and keeping track of your efforts is both fun and educational. Enjoy yourselves and keep us informed of your progress too.
The TC ULTRA handles are excellent for this type of flight dynamics. The holes in the plexiglass make it easy to stake your kite down under tension in a specific set-up position. The handle has a finger outline like a pistol grip that easy to rock against, forward or back. Since your not on Rev handles you don't immediately have preconceived expectations on "feel" either. You are going to be making new muscle memories, start fresh everywhere!