[U-Boot] Hoping to dual-boot an M8S Pro / Amlogic S812 machine

Hello everyone, happy holidays,

I want to dual-boot Gentoo and the original Android on one of these https://www.amazon.com/NinkBox-Android-Amlogic-Bluetooth-streaming/dp/B01EOGQ3CE boxes. To install Gentoo, I found an equivalent tutorial for a box named TrimSlice, here https://wiki.gentoo.org/wiki/TrimSlice. Nowhere in that tutorial do they actually make the target box dual-bootable, so that is my question:

How do I make that M8S Pro dual-boot with U-boot? I don't want to nuke the original Android off it.