Experimenting with dual booting various Operating Systems on devices like the Steam Deck can be a pain. However, I would like to point you at this GitHub Repository called “Steam rEFInd”.
What the github repository contains is a installer that installs a open source bootloader to your Steam Deck’s boot partition. This allows one to be able to dual, triple, or quadruple boot different operating systems.
Operating systems supported include SteamOS/Linux and Windows, either installed on the internal NVMe drive, microSD, or optionally, a USB 3 device.
Setup is a little involved and requires you to be comfortable with the linux command line; however it will try to guide you as much as possible.
Install, configure, boot all the things. Job done.