Merge windows 8 32 bit and 64 bit Dart as one iso.

1) get gimagex v2
2) export the boot.wim index 2 from your x86 and x64 versions to a new boot.wim
3) make a shell iso directory by extracting an appropriate win8.x iso to a folder such as c:\win81
4) open admin command prompt and navigate to the folder you just made. Then go into the boot directory. Ex: c:\win81\boot
5) type this command and note the GUID it mentions that it copies to:

Code:

bcdedit /store bcd /set {default} bootmenupolicy Legacy
bcdedit /store bcd /copy {default} /d "x64 DaRT 8"

Now run these 2 commands and replace the {GUID} with the actual GUID in {}'s:

Code:

bcdedit /store bcd /set {GUID} device ramdisk=[boot]\sources\boot.wim,{7619dcc8-fafe-11d9-b411-000476eba25f}
bcdedit /store bcd /set {GUID} osdevice ramdisk=[boot]\sources\boot.wim,{7619dcc8-fafe-11d9-b411-000476eba25f}

If you want to change the original default description you can try this:
bcdedit /store bcd /set {default} description "Your Description Here"

It's been a while since I've messed with descriptions. You might have to tweak that command a bit.
I haven't done much tweaking with UEFI multi-boot stuff. You might not be able to do that with boot media like this. I think that UEFI installs boot options into the motherboard's bios.
I really am not entirely sure of the do's and don'ts of UEFI multi-booting. On top of that you'd have to repeat these things for the c:\win81\efi\microsoft\boot\bcd file.
Plus you have to make the target media UEFI bootable. It's actually kind-of confusing. Basically, don't try to make a multiboot cd/dvd for UEFI use.

6) If you're not using a USB flash drive, you'd need to make your ISO bootable. If you use oscdimg.exe, it would go something like this:
oscdimg -u1 -lYourLabelHeremax32chars c:\win81\ c:\temp\YourOutputIsoName.iso

Again, if you want it to be UEFI bootable, it's a lot more complicated.

*EDIT: actually UEFI is not much more complicated.
You simply edit the efi\microsoft\boot\bcd file using the bcdedit instructions above except make sure it only boots into x64 versions of the boot wim
Then copy bootmgr.efi and bootx64.efi to the respective directories from an x64 iso. This will kill the ia32 efi booting, but I've only even heard of some tablets using that.