How to switch between two different Windows OS located at separated physical disks?

I’ve added new SSD into PC in order to install new OS (Windows) and switch between with boot menu.

I load boot menu while computer is powering on, choose the old disk and loading to the old OS.
Then I reboot PC, choose new SSD and load into the old OS again.
The only way to load new OS is to disconnect old disk from motherboard.

I was told that the issue is there’s two separate loaders for two disks, and I need to make common loader for both of them.
How to do it and don’t damage old OS?