I missed your question, but I've done that many times. I don't recommend it unless you're really careful. On most surfaces you'll still want to either: 1) nail a brace to the wall you're working on, C-clamped or whatever to one or both of the stepladders; or 2) prop the stepladders with a long 2 x 4 from behind. I put a stake in the ground, nail the 2 x 4 to it, and clamp the 2 x 4 to the stepladder. I use two braces, one to each stepladder.
Using stepladders that way has long been a way to set up a low scaffold, but you'd better have good balance and not try to get too high with it. It does work, however. Using the braces slows the whole process down. It's a question of how you feel about broken bones.
I've also built homemade scaffolds and it's a real PITA, in my opinion. I use 2 x 4s for the verticals and 1" electrical conduit for diagonals. Never count on those diagonals to handle compressive loads; use two, crossbraced, so the load is always in tension.
The slickest solution I've used is two regular ladders with ladder hooks for a scaffold plank. I'll go up about ten feet with that, but no higher. Again, you want to nail a brace to the wall. Use a short plank or else make sure you're using genuine scaffold plank, which is undressed and thicker than framing planks.
-- Ed Huntress