I always get 2.6 at http://www.portableapps.com
Never had a problem yet and yes it should be no problem running them together on separate USB drives.
I never tried it with both.
On the PC it shouldn't matter.I've use both in one folder on my desktop named Gimp with no compatibility issues before.
I am 2 seconds away from uninstalling all but SamJ's Portable Gimp-2.8
It works so perfectly for me.If you can not find it let me know and i will upload one for you (same one i got from there)http://220.127.116.11/lhfbabuwua1g/3m1jz ... 12.paf.exe
PortableApps only has 2.8 currently (can't locate their archives, assuming they have such a thing). Downloaded your version as backup, but since this isn't for me, I'm stil looking for a trustable public source.