... and? If one engine works entirely fine on a 100% blend, with no ill effects in operation or in analysis afterwards (for weird deposits on the hot section),

... and? If one engine works entirely fine on a 100% blend, with no ill effects in operation or in analysis afterwards (for weird deposits on the hot section), it's reasonable to conclude that one could run exclusively on it. There's a chunk of wisdom over in experimental aviation (the "You can build and fly whatever you want, please try not to kill anyone else in the process..." side of things): "Never test an experimental engine on an experimental airframe." If you're building a new airframe, use a proven engine. If you're trying a new engine, use a proven airframe. Otherwise you just end up with an explosion of possible issues, and "experimental engine plus experimental airframe" tends to lead to an alarmingly short lifespan. Running one engine on an experimental fuel, while keeping the other on something you know is entirely sane and will work fine, is entirely reasonable. You know the airplane can fly and land safely on one. You think the other one works fine - but there are some weird corner cases in aviation - British Airways Flight 38 was one of those things. In this particular, exceedingly difficult to reproduce case, water in the fuel iced up and clogged the fuel flow to both engines, leading to a crash short of the runway (no fatalities, mostly due to the skill of the pilots). The engines behaved absolutely fine, until short final, at which point they suddenly, and without any prior warning, refused to make power - because they couldn't get enough fuel. The entire point of testing is to work out as much of this sort of stuff as you can. If it turns out that after cruise, you've got weird fuel icing issues, it is far better to have that problem limited to one engine, where the other one can pick up the slack and haul you into a goaround if needed (twinjets are typically obscenely overpowered on two, because they need to perform well with an engine out). Doing it the way United did is entirely sane, reasonable, and rational for any sort of aviation testing. Doing it on both engines, without serious testing beforehand (of the "on one side" variety), would be insane in aviation.