Question – When and how to set up a sufficiently large EFI partition for dual Secure Boot?

Best way is to create the uefi partition on a clean disk, if it is the first partition on the boot disk then the system will always boot from that, Windows will pick up on it and automatically add itself to the boot menu, for linux I’m not sure, it does ask you at some point where you want to install the boot loader, if you don’t let linux install any bootloader you can add the linux OS to the boot menu by using either the recovery console of windows if you want to do it manually or by using easybcd from windows.

You also don’t need a boot partition, windows can have the boot files on the same partition as windows, then you only need to create a partition for linux, don’t let it install bootloader and add it with easybcd.

I’m sure you can do the same thing with grub or whatever linux uses for uefi, I’m just not familiar with that side of things.