From my experience, separate them. When I got around to blowing my windows installation away it was a pain in the ass to switch the boot to another partition.

Make two partitions. If you only have the one, windows thinks it owns it and will randomly overwrite your EFI stuff.

If you make an extra partition (extra drive is advisable too) then install Linux on the new EFI then part of the install should probe foreign OS and add a chainloader entry to your Linux grub boot. In BIOS set your Linux boot entry as the default. When it boots you will have the windows option show up besides the linux one, which will handover the boot process to windows at that point. This way windows is unaware of the Linux boot partition and doesn't know it was chainloaded. It will leave your EFI partition alone. Just when you initially go back to windows if it gives you a desktop notice saying partitions found/assign drive letter etc just click ignore/do nothing. Don't assign drive letter. Windows will then ignore the extra partitions.