To close the circle on my little experiment...I thought both parts of the split batch were done, with the Nott at .009 and the ESB at .012 (I had mixed it up in my prior post), so I checked each for flavor. There was a noticeable difference, with the ESB tasting a little fruitier. But the next day, the ESB started up again in an odd way. It got very cloudy and had pinprick sized bubbles surfacing. This went on and on for some days. I worried about a wild yeast strain, but was dreading that somehow some brett had made it in from a kriek I had brewed in 2017, even though I like to think I rigorously sanitize. The most likely possibility was that somehow I crossed contaminated the ESB portion with yeast from the Nott portion, most likely when I was tasting, although I am very careful about contamination, always using an isopropyl spray and swabbing the neck of both carboys with separate paper towels laden with the isopropyl spray.
Losing patience, I just kegged it all together; sometimes you just have to sail into the unknown. WWCD? (What would Columbus do?) Two weeks later the beer has just about cleared and I have a very flavorful ordinary bitter of about 25 IBU and 4% alcohol. Nice, yeasty, fruity nose and a bready sort of flavor. No errant wildness from renegade yeast, and thankfully, no brett. It's a simple and plain beer reveling in its unsophisticated youth, a happy outcome. Two things I don't like about it: It has a lot of head and the carbonation leaves solution very quickly, but I'm pretty new to kegging, so my inexperience might be affecting things there.
Lessons to share: 1) Marris Otter is great. 2) The kegged outcome was similar to pitching Nott for a drier finish after having let the ESB get a jump on things to capture the ESB fruitiness, but I just went about things indirectly. 3) No matter how careful one is with sanitation, to err is human, but yeast is pretty forgiving. 4) If I'm going to split batches with different yeasts in the future, I'm going to keep the fermenters in separate rooms and not use the same sanitation tools on both fermenters. 5) Get a fridge.