Hold the volume down button while booting to get to the boot menu. If you get a bootable drive connected, it should show up there. You could connect your windows ssd and it should work fine.

However, the steam deck needs its drivers for the GPU, Bluetooth, and similar. If you plan on swapping back and forth, that may not play nice. Installing portable windows to an SD card really ended up not being that hard and I’d be happy to link you to a guide if you needed one.