×Working backwards from the end, I have to kill the last adder with the decoy. It can deliver one stab before frying, so the adder must be 2 tiles long. The problem is that trapping an adder either by itself or the other adder, while its head is on a hot tile, shortens it by two segments per turn, and they start with an odd length. So I think I have to use snake 2 to trap snake 1's head on the trapdoor and shorten it by an odd number of tiles, then dispose of snake 2, then use remaining even-length snake's own body to reduce its length to two.

But I can't do the first step, they are really awkward about going west. The best I can do is trap one's head on the trapdoor and kill it there, but that's clearly not enough.