No, two PE images don't require separate partitions.
You must have /boot/bcd /EFI and /bootmgr. Place your PE .wim files somewhere on the drive (I just keep all three of mine in /sources and name them differently. Some .wim files have two parts - such as my win7 universal installer. DON'T rename the install.wim!)
Download EasyBCD.
Open it and click file>select BCD store, point this to /boot/bcd on your multipass
Now you are able to add all of your .wim images to the bcd menu, via "Add new entry" and selecting "WinPE" under "Portable/External Media". Just point each entry to a .wim file. I've currently got three different PE images and one extracted XP OS all booting from my bcd store, which I'm chainloading from grub legacy (via chainloader /bootmgr)