How does one make a layer cake from flourless chocolate cakes?

I probably wouldn't do so as you mentioned flourless chocolate cakes typically have a dip in them, a liquid center or are otherwise very delicate. It's hard enough to get a single flour less chocolate cake to be presentable - layering them on top of each other seems like a disaster waiting to happen. I'm not saying it can't be done, but chances are good that you'll have trouble. What's the idea behind using flourless chocolate cake? Are you looking for the same flavor? Same texture? A liquid center? I'd research recipes for chocolate cakes that match the flavor you want and use those more stable approaches to make your layer cake. You can mimic the flavor by using good ingredients, a good recipe and good technique. I'd look at books like Joy of Cooking, Baking with Julia, the Cook's Illustrated's book on Baking for tips and ideas. The Joy of Cooking book I find is good because it explains the hows and why's. And with that bit of knowledge you'll understand why you use flour and why not and what the effects will be on the final product. You might also look at some books like Maida Heatter's 'Great Book of Chocolate Desserts' and Nick Malgieri's 'Perfect Cakes' for more ideas, and inspiration. Good luck!