I have one for each camp now and I was wodnering, what advantage does proof give me? Can I buy the light and dark equivilents if I have a token from each?
The more "proof" (faction) you have, the further into DoN progression you can get, and get bonus AAs that do stuff such as +5 max cap on stats, 250 hp, mana, etc etc. More details are in the quest links above. There is absolutely no advantage one has over the other. You can also buy from the other camp at max KOS.
When you partake in a solo task or mission for that particular camp, only that camp will you improve your standings upon completion Tokens are not interchangeable. For what items i looked at from top to bottom at either sides merchants the augs and equipment are idenitical in stats and list ALL as races that use the items, only differences i see are the names and looks of of those items. I'm working toward indifferent on the Norrath's camp for my orge to aquire a discipline and some augs, of which the dark has also.
There's one merchant to each camp which are located in the tunnels leading to Bloodlands.