I have been testing this to see what might be going on. If you install Windows 7 in the UEFI configuration, which means Secure Boot is not enabled. Then you install Windows 10 using UEFI in Dual Boot mode, you end up with a Boot GUI with selections for Windows 10 and Windows 7.
If you select Windows 7, you will get the screen below, so you have lost the boot capability to Windows 7. Since Secure Boot is not enabled, I have to assume the Windows 10 boot manager will not recognize the Windows 7 EFI OS loader.
There are probably not a lot of folks running Windows 7 as UEFI, but something to keep in mind with the release of Windows 10. I have not tried installing Windows 7 after Windows 10 so not sure what it might do, assuming you could even boot the Windows 7 install media in UEFI mode.