> Booting two different operating systems is just fundamentally at odds with secure boot...
Agreed with your first sentence but not really with the second. The idea is simply securing the boot process of an operating system from POST right through to the OS being ready for use. Having 2 operating systems is perfectly fine from that point of view as long as the integrity of the boot process can be maintained. Having a boot loader and 2 or more OS's available complicates things, but doesn't make it at-odds with secure booting.
Edit: Manually correcting autocorrect and adding a couple of words
Agreed with your first sentence but not really with the second. The idea is simply securing the boot process of an operating system from POST right through to the OS being ready for use. Having 2 operating systems is perfectly fine from that point of view as long as the integrity of the boot process can be maintained. Having a boot loader and 2 or more OS's available complicates things, but doesn't make it at-odds with secure booting.
Edit: Manually correcting autocorrect and adding a couple of words