I have written a new Tutorial 127 here which describes how to make a WinPE dual-boot payload that will boot from a 32-bit or 64-bit UEFI system or a normal MBR BIOS system.
Although the Win8.1SE project by ChrisR is supposed to have this Multi-WIM capability, I found that I could not get it to work without having to use BootIce to fix the BCD, so it was easier just to manually merge the files from two WinPE's into one.
This shows just how easy BootIce is to use for simple BCD editing!
Once you have made the FAT32 USB drive, you can make a partition image from it using MakePartImage and add the image file to your E2B USB drive.
Although the Win8.1SE project by ChrisR is supposed to have this Multi-WIM capability, I found that I could not get it to work without having to use BootIce to fix the BCD, so it was easier just to manually merge the files from two WinPE's into one.
This shows just how easy BootIce is to use for simple BCD editing!
Once you have made the FAT32 USB drive, you can make a partition image from it using MakePartImage and add the image file to your E2B USB drive.