I was wondering a few things.
- Is this a new addidtion? Do both 841's have the same firmware?
- When you change the voltage options does either machine operate differently? Have you tried just the 1 machine on it's own for a baseline?
- Are you just basing off the GHSasc numbers? If so I think that is just a peak hashrate reached not sustained.
- Can you post a copy of your API log? Might be something in there.
I remember with the 741's there were some lottery units based on their chip design that just outperform other units, not sure if this is true for the 8 series. You may have gotten lucky if there are no adverse affects. I noticed you said it slows down after reaching the high temperature limit, does it ever shutdown?