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Dual booting Linux distros using images sourced from USB drives

I have 2 USB flash drives each with a bootable Linux distro.

I believe that I can use Gparted to copy each of them onto separate partitions on a single (clean) SSD.

USB drive 1 with Linux 1 -> SSD partition 1 (40 GB)
USB drive 2 with Linux 2 -> SSD partition 2 (80 GB)

I am unsure though how I should set up the SSD – formatting, required partitions (boot, root, etc.). Also, after I copy the images to the SSD how would I be able to choose which Linux distro I’d like to boot into?

This post seems to cover most of what I’m looking for but is focused more on installing the second distro:

https://askubuntu.com/questions/1329599/dual-boot-two-linux-distributions-on-one-disk

Note that I don’t have the installers for my distros. Each is custom configured and I’d prefer to just copy them from the USB drives to the SSD as-is. Also, I don’t have UEFI.

Thx.