Yep, this is what I do too. Especially for swimming; like in the long vertical section in 8-3. I use my thumb for both buttons.

I use the button layout of B/Y for run and A/X for jump, and I use my thumbs for everything and hold the DS normally.

Does anyone have an alternative for this? I have X/Y for run and A/B for jump, and I used to do the alternating A-B mash with my thumbs. It’s fast, sure, but my thumbs aren’t big or wide enough to keep it going(they usually cramp up either in the first room of 8-2 or the vertical section in 8-3) so I have to hold it differently in those sections

I switch my grip for mashing in 8-2 and 8-3 so I can use 2 fingers to mash, allowing me to mash fast