Well, just for the fun I made this:
60 GB USB mechanical HDD MBR partitioned:
First partition: Primary active 1GB Fat 32 (Large for future test * ): Boot files for MBR and EFI/UEFI
Second partition: Primary 20 GB NTFS for Windows.
Third partition: Primary 37.x GB NTFS for install.wim (wimboot mode)
First Test: Dual Boot MBR & UEFI
Using synchronicity wimlib-imagex v1.80: http://reboot.pro/to...implementation/
and ReTokener tool wimlib_clc_150319: http://reboot.pro/to...ta/#entry191777
Installed 8.1u1x86 to second partition, boot files on first partition.
I captured a 8.1u1x86 system fully updated and syspreped (from second partition) in wimboot mode to install.wim image (located in third partition, same HDD).
Then formated second partition and apply captured install.wim wimboot mode to it.
It booted without any problem in MBR/legacy, also it is seen as a boot option in UEFI (haven't run it but I see no problem).
Second test: 5 Booting Options = Dual UEFI Boot (x86 & x64), & Triple MBR Boot (x86, x64 & grubfordos)
Later I formated first, second and third partition and, using install.wim x64 and install.wim x86, directly from install Iso's, just extracted to third partition, each one in a diferent folder. Then created in second partition two 5 GB VHD's first for 8.1x64 and second for 8.1x86, both wimboot mode install.
To make the wimboot install to VHD's I used JFX tool WinNTSetup v3.77 & libwim-15.dll from wimlib-imagex v1.80 (in WinNTSetup3.77\Tools\x86 or x64 \wimlib), using as source each image in third partition, and selecting first partition as boot one, (WinNTSetup makes all changes you need for booting x86 & x64 in MBR or UEFI, also the required VHD's in the selected partition).
Later after add grubfordos to the BCD using BootIce I can run x86 or x64 and also grubfordos (to run some Iso's) just selecting it from the first window, when in MBR/legacy.
Or x86 and x64 in UEFI (if machine can use both, some do, other don't), but NOT grubfordos in this case unless deactivating UEFI and Segure Boot.
I'm sure it is also possible to make a dual full install (not wimboot), if you install x64 in one partition and x86 in the next one, as long as the first partition (Fat32 format) has the boot files for both.