I have returned.

I poked around a bit for double sixth scale fingerings, but I wasn't finding anything, not like I found for scales in thirds.
This looks interesting, for the fingerings, and also because they just copy and pasted something there. And then they're probably selling something on top of that. I'm wondering what the source is. And after looking at this post, I wondering what happened the Maclaren book I got... and if that had these fingerings. Or it's likely something else had them that I've got.
https://bluesjazzpiano.com/major-sixths-scales.htmlAlso found this but I haven't looked at it yet.
https://www.wheaton.edu/media/migrated-images-amp-files/media/files/academics/faculty/horn-dp/DPH-FINGERINGS-FOR-DBL-6THS.pdfJust for C Major, it feels like my left hand isn't quite as stretched out/usable as my right, which makes sense. The 25 14 jumping makes that obvious.
What I did find was the old solfege line works well for getting the fingerings down on these. I only spent a few minutes on them yesterday and today. Five minutes on RH yesterday. Five minutes on LH today. Enough to pick out the patterns, but my hands don't have them down yet.
These patterns work well.
Do.
Do re do.
Do re mi re do.
Do re mi fa mi re do.
Do re mi fa sol mi re do.
Do re mi fa sol la sol fa mi re do.
Do re mi fa sol la ti la sol fa mi re do.
Do re mi fa sol la ti do ti la sol fa mi re do.
Although it breaks the pattern of staying on the beat by repeating do between them.
Do.
Do ti do.
Do ti la ti do.
Do ti la sol la ti do.
Do ti la sol fa sol la ti do.
Do ti la sol fa mi fa sol la ti do.
Do ti la sol fa mi re mi fa sol la ti do.
Do ti la sol fa mi re do re mi fa sol la ti do.