Dual boot on NVME SSD

The context in short:

I have a Rock pi 4B, i’m installing Debian on my nvme SSD (a Kingston A1000, not compatible with spi loader). so i’m rely on mSD for start the boot on NVME (and later with a eMMC module).

So i’m srinked the Debian partition on nvme, and create a new one with recalbox on it.
But now i’m need to have a way to boot on the recalbox partition or Debian partition, a multiboot.

But i’m not known how doing this: How have a multi boot who can start Debian or Recalbox ?

when i’m received my eMMC module, i’m plan to have Debian on it with home mounted on a partition on SSD, and recalbox on another partition on SSD nvme.