This is the most deadly and awesome concept (1 legendary miner/mason and 5 peasents already died draining magma to redesign it). It is based on the obsidian machine-gun. But it doesn't work at all . Any ideas?
It's hard to imagine exactly what's going on without drawings.
It sounds like you have offset water pumps and magma pumps set to output to the same tile, but only steam is being created, and only on one side of the trap?
So the fact that it's working differently on either side suggests that you built each side slightly differently than the other. That's to be expected-- build order is a concern, and you can't build things simultaneously. For instance, if you have two pumps feeding from a single square, and they're both always on, the most recently built pump will always get all of the fluid, and the pump built first won't ever get any.
The one video I saw of a cave-in trap (I trust video first, screenshots second, forum reports third) involved water and magma spilling out from two bridges and meeting in a single square between the bridges. I have read reports from 40d where the author (eli dupree? i think) claimed that water and magma won't obsidianize while both are falling (although I've heard more recent reports denying this). If I wanted to do this and was tired of experimenting, just wanted to get it done, I would design a system where the pumps fed simultaneously and continuously onto two bridges (to permit cave-ins) rather than directly on top of the channel.
I'm not sure which method you're using to offset the water and obsidian pump. If this offset is too small, one fluid will not have fallen before the other pumps. Depending on your exact reasoning, it would seem to make more sense to me to pump directly into your channel with one fluid, and drop the other fluid from above-- over-under pumps, rather than facing pumps.