I've done a few decoctions, but wasn't happy with the results (It's not the decoctions fault, I just suck at brewing). For simplicity and flavor, I much prefer a 90 minute boil, but I haven't completely given up on decoctions yet... I just don't have any plans to do another one any time soon.
As for recipes, I can't help you. I've never done a dubbel. I want to. I've spent a lot of time thinking about it. I have an AG recipe which I may use to make a dubbel. But that requires an extended brewing session and I don't seem to have much time these days.
Anyway, if I were going to formulate a recipe, I would work in reverse. Start out by figuring out how much grain my MLT could hold for the second runnings. Divide that amount of grain by 2, 3, or 4 (for a dubbel, trippel, or quadrupel).
I would then use a smaller vessel such as the Mini MLT presented on this board (run a search to find it), to make my sets of first runnings, then dumping the grain into the big MLT. FWIW, I would think this could all be done on the stove since you're not boiling anything yet. I would aim to collect enough for 2 to 4 gallons, depending on the size of my main MLT and how much grain it could hold.
Boil, Cool, pitch it on a yeast cake, and let it go. Then worry about what to do with the second runnings. Of course, that will depend on the strong beer recipe.
But come to think of it, I would probably design the light beer first... and let that dictate the quadrupel. Why not do it that way? You're in such a rarified stratum of brewing, my opinion is that you should almost forget about cloning and just have at making something truly unique.
Make your own recipe. That's what I say.