Configure Windows Boot Manager

Add it to Windows Boot Manager

Open Command prompt as Administrator. Type:

set "MainOS=<MainOSLetter>:"
set "WinArm=<Windows10Letter>:"
set "id={703c511b-98f3-4630-b752-6d177cbfb89c}"
set "bcdLoc=%MainOS%\EFIESP\EFI\Microsoft\Boot\BCD"

Where <MainOSLetter> is the drive letter of MainOS. <Windows10Letter> is the letter of Windows 10 ARM partition.

For example:

set "MainOS=E:"
set "WinArm=F:"
set "id={703c511b-98f3-4630-b752-6d177cbfb89c}"
set "bcdLoc=%MainOS%\EFIESP\EFI\Microsoft\Boot\BCD"

Press enter. Then type:

bcdedit /store "%bcdLoc%" /create %id% /d "Windows 10 ARM" /application "osloader"

bcdedit /store "%bcdLoc%" /set %id% "device" "partition=%WinArm%"
bcdedit /store "%bcdLoc%" /set %id% "osdevice" "partition=%WinArm%"
bcdedit /store "%bcdLoc%" /set %id% "path" "\Windows\System32\winload.efi"
bcdedit /store "%bcdLoc%" /set %id% "systemroot" "\Windows"

bcdedit /store "%bcdLoc%" /set "{default}" description "Windows Phone"
bcdedit /store "%bcdLoc%" /set "{bootmgr}" custom:0x54000001 %id%
bcdedit /store "%bcdLoc%" /displayorder %id% {default}

bcdedit /store "%bcdLoc%" /set %id% "testsigning" Yes
bcdedit /store "%bcdLoc%" /set %id% "bootmenupolicy" "Legacy"
bcdedit /store "%bcdLoc%" /set %id% "detecthal" Yes
bcdedit /store "%bcdLoc%" /set %id% "winpe" No
bcdedit /store "%bcdLoc%" /set %id% "ems" No
bcdedit /store "%bcdLoc%" /set %id% "bootdebug" No
bcdedit /store "%bcdLoc%" /set "{bootmgr}" "nointegritychecks" Yes
bcdedit /store "%bcdLoc%" /set "{bootmgr}" "testsigning" Yes
bcdedit /store "%bcdLoc%" /set "{bootmgr}" "timeout" 5
bcdedit /store "%bcdLoc%" /set "{bootmgr}" "displaybootmenu" Yes

Press enter. Please keep this command prompt for the next step.

Finishing touches

This step is also important. If you don't do it, Windows Setup will fail.

Type in the Administrator command prompt:

md %MainOS%\EFIESP\EFI\Microsoft\Recovery\
bcdedit /createstore %MainOS%\EFIESP\EFI\Microsoft\Recovery\BCD

Press enter.
Then type (line by line):

diskpart
sel vol <MainOSLetter>:\EFIESP
set id=c12a7328-f81f-11d2-ba4b-00a0c93ec93b
exit

Replace <MainOSLetter> with the drive letter of MainOS.

At this point, you are ready to move on to the next part.

The next part: Boot and setup Windows 10 ARM