I find I enjoy helping other OS X self builder's and enthusiasts with their systems after I've been helped many times myself by this very community. This is a "straight to the point" guide, not "how I did it guide".
HERE WE GO
This guide has no guarantee it will work for you as it did for me. Use this information at your own risk.
This all inclusive guide is targeted for helping others using UEFI booting of Windows 10 alongside their OS X using the Clover boot loader 'UEFI' option installed on the OS X partition through latest EL Captain Multi-beast version 8xx. If you're using Clover legacy boot settings this guide won't be for you . . . .
so again --->'Not For Legacy Booting' nor older motherboards that don't have ability for UEFI booting.
It is most important that your bios settings are all properly set for UEFI booting. tonymacx86 has plenty of members in this community that have already posted guides to explain how to properly set up your Bios for UEFI boot so I won't be spending any time explaining this.
THE PROBLEM:
You post installed CLOVER UEFI boot loader to your OS X partition, achieved booting OS X UEFI but Windows 10 you can't.
THE DESIRE:
You want the holy grail. The same ability to boot your Windows 10 from the Clover boot loader just like OS X.
HERE'S HOW:
Attached bottom of this post is a zip file containing the proper configured Windows 10 UEFI contents for adding to your Clover boot installation within your permanent OS X partition. Download the zip file and open it. Once zip file is opened reveals the main folder. Inside you'll discover 2 objects. #1 A folder labled 'Microsoft' and #2 a Windows BOOTX64-win.efi file. I'll explain later in steps #9 and #10 for exactly where to place these items. Once completed you should have no problem booting Windows 10 from your Clover boot loader UEFI installation equally without hiccup just like booting OS X from that same Clover UEFI installation.
This guide designed for simplicity of how to rather an explanation as to why.
1. Boot into Windows 10 directly by using keyboard hot key such as F8 or F12 (you likely require the bios boot settings being switched back to the legacy boot options 1st in order to do this.)
2. Once booted back into Windows 10. Open command window 'run as administrator'
3. enter command: bcdboot C:\Windows /s C: /f uefi
4. You should then see a following message "Boot files successfully created". 1st step success!
5. Restart computer, enter bios & reset the bios legacy boot settings back to UEFI boot settings if need be.
6. Clover boot options should now be back. Now boot into OS X El Captain or your whatever OS X installation.
7. You now should see a Windows 10 drive and possibly a reserve drive mounted on the OS X desktop. Leave them there.
8. Using an EFI mounter like 'Clover configurator' mount OS X 'EFI' partition. Follow next 2 steps as specified.
9. Open the mounted 'EFI' drive/folder on the OS X desktop then simply drag the attached entire 'Microsoft' folder inside that 'EFI' folder. Don't mess with any contents within 'Microsoft' folder. It is configured to boot Windows 10 UEFI. Just drag the folder as specified.
For those who are still newbies learning your way around OS X and have attempted to mount your EFI drive/folder but do not see it mounted on your desktop try this:
A. Open Finder (smiley face square icon bottom of your desktop)
B. Now left click the actual word 'Finder' located at top left corner of your desktop for revealing a list of selactable options.
C. Select Preferences
D. Under 'show these items under desktop' make sure 'Hard Disks' is checked (EFI drive should now be seen mounted on desktop)
10. Now take BOOTX64-win.efi and drag that file into the 'BOOT' folder sitting inside underneath same 'EFI' folder. You now are completely done.
11. Other Points Need Mentioned to avoid mistakes. Underneath 'EFI' folder make sure you put nothing in the Apple or Clover folders. Leave them alone. The Microsoft folder should now be seen listed or being grouped among APPLE, BOOT & CLOVER folders under 'EFI' after you dragged it there. The only file you'll put within another folder is placing BOOTX64-win.efi inside that 'BOOT' folder sitting inside the "EFI' folder.
12. You can unmount Windows 10 or any mounted partitioner for that matter on the OS X desktop simply by right clicking then selecting 'eject' from the list.
13. Hopefully now a restart back into the Clover provide you booting options either OS X or Windows without problem.
I hope this simple explanation for multiboot of OS X and Windows works for you. God Bless.