OK, here we go.
You'll need to have ready your 2 iso's. One for the Ghost 15 recovery disk and one for your Ghost 12 recovery disk.
Rename them to Ghost15.iso and Ghost12.iso while they are still on your computer, renaming them later can cause them to fragment and they need to be on the flash drive as one continuous file without any fragments.
Make a folder on your flash drive called "Portable Apps" inside that folder make another one called "Defraggler"
Go here and download to your desktop "Defraggler Portable" It's a free program but make sure you get the portable version.
http://www.piriform.com/defraggler/builds
Extract that zip file to your flash drive into the Portable Apps/Defraggler folder
Copy the Ghost 15 recovery iso to the root of the flash drive (not into any folders).
Then open the Defraggler folder and run "defraggler.exe"

http://img218.imageshack.us/img218/5980/defrag1.jpg
On the top, select the flash drive and then click the "Analyze" button.
When it's done, select the tab that says "file list"

http://img5.imageshack.us/img5/3200/defrag2m.jpg
If you see the Ghost15.iso listed, check the box next to it and then select "Defrag". It may take a while to defrag it.
If it's not listed, it's not fragmented.
Next repeat with the Ghost12.iso, copy it to the drive and run defraggler again and defrag the iso if necessary.
Trust me, it's quicker to do them one at a time then copying all the iso's onto the drive and finding you need to defrag any of them. If you have troubles and your flash drive is small, you may want to delete the iso's, defrag the flash drive and copy an iso back to the drive again.
Once you have those iso's on the drive without any fragments download grub4dos here:
http://sourceforge.net/projects/grub4dos/
Extract that to a folder on your desktop and copy these files to the flash drive:
grldr, grub.exe, and menu.lst
Open up menu.lst in notepad and delete everything and replace it with this:
-----------------------------------------------------------------------------------
default 0
timeout 30
color cyan/blue white/blue
foreground ffffff
background 003f7d
title Start Ghost 15
map (hd0,0)/Ghost15.iso (hd32)
map --hook
chainloader (hd32)
boot
title Start Ghost 12
map (hd0,0)/Ghost12.iso (hd32)
map --hook
chainloader (hd32)
boot
title Boot to DOS
root (hd0,0)
chainloader (hd0,0)/io.sys
title Reboot
reboot
-------------------------------------------------------------------------------------
Don't copy the lines m8, just everything in between them.
Save it and close notepad.
Open up disk management and look for the flash drive. In the bottom section see what Disk# is listed for it. You will see the drives listed as Disk 1, Disk 2 etc.
My flash drive is listed as Disk 4.
Download and extract this file to a folder on your desktop:
http://www.themudcrab.com/downloads/grubinst-1.1-bin-w32-2008-01-01.zip
Run the file grubinst_gui.exe

http://img513.imageshack.us/img513/192/grubinstall.jpg
In the top box select the drive number and double check the size listed to confirm it is your flash drive.
BE VERY CAREFUL AT THIS STEP- You must be sure it is your flash drive, if your wrong here you will cause your system not to boot.
If your sure it's your flash drive, click the setting "dont search floppy:" and click "Install"
A command box pops up saying it has installed, you can close everything now.
Boot to the flash drive and Grub will load the menu.lst showing the boot choices.
When you select one of the iso's it will be just like booting to a CD, you'll have to "press any key" to boot to the "CD".
When you boot to DOS if you want to start Ghost 2003, just type: ghost
If it all works you now have a multi-booting flash drive with all your versions of Ghost to choose from.
Dave