I've recently got my hands on one of those SanDisk Extreme 64GB sticks and started tinkering with it. During this story, please keep in mind that this is a model where removable bit is set to 0 (which means Windows sees all its partitions by default - each is shown as a single disk in explorer).
To have multiple functioning Windows installers on a single partition. This either means getting the installer to function from .iso image without unpacking (this is the way which is being pursued in this thread) or editing BCD in such a way to allow installers co-exist on a single partition (alternative way, secondary to the first one).
Now originally this all started as a question  on SuperUser. This was also discussed on RootAccess  chat room. This is where the link  was discovered. After digging it for 4 straight hours, I was able to successfully get to the screen of Windows installer (this was all attempted on Windows 7 installer but it seems Windows 8 installer is no different - correct me if I'm wrong) where you need to select (create, delete, modify) disk partitions.
Simple Windows .iso was used as a starting point for customization. It's boot.wim (\sources\boot.wim) was modified to have ImDisk driver available for installation at the time of running setup.exe. The following commands are used to make our .iso available after switching to protected mode (during setup.exe):
rundll32.exe setupapi.dll,InstallHinfSection DefaultInstall 132 .\imdisk.inf imdisk -a -f "%%G:\multiboot\ISOS\Win7.iso" -m #:
First line installs the ImDisk driver.
Second line makes our .iso available in form of UDF drive (essentially a CD-ROM).
This script is tested and working (since we are able to select Windows version for installation). It can either be run manually (to be sure what exactly is happening and when) or automatically by using Winpeshl.ini. The modified boot.wim is written back to the .iso file by using WinISO. After that, YUMI  is used to boot this iso from the second partition of SanDisk Extreme (using GRUB .iso boot).
When selecting a partition for Windows installation - a message is displayed saying that 'installer was unable to create or locate the existing system partition'. Accessing the setupact.log during this time reveals that disk0 (the fully empty laptop internal hard drive) was skipped due to not being 'the computer's boot disk'. This same error is described in article  (last one):
[BLOCKING reason for disk 0: CanBeSystemVolume] The selected disk is not the computer's boot disk.
1. Making the second partition on SanDisk non-bootable. SYSLINUX's altmbr.bin was used to boot second partition by number - not by boot flag. Same issue.
2. Writing Windows installer to another boot stick with unpacking - the usual easy case. When having both sticks plugged in and trying to install from disk with unpacked installer does NOT result in error (although SanDisk is obviously still detected as non-removable hard disk).
1. Not sure why this error is even displayed. How in the hell is it able to determine if it is the boot disk or not. Why in the hell would it even care .
2. How can I make it forget this nonsense?
3. Maybe there is altogether different way? Like custom installer described here .
Custom installer 
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Edited by Gear54rus, 22 April 2014 - 12:04 PM.