Crap, I was hoping that I could create some kind of trading post, but my idea requires that the trading post in (Sanctuary) can talk to the trading post in (red rocket).
I'm just going to splat my solution here, just in case I can inspire someone with actual skill to pick up the idea and run with it. A building that has an adjustable amount of water and food that is connected to every other "trade post" like building. When one building takes water and food away from its settlement, it goes into a pot shared by all the other tradepost buildings. Then in other settlements they could adjust the water/food upwards on a 1 for 1 basis.
The Trade Post in Sanctuary could look like this (Food +3 Water -10)
The Trade Post in Abernathy could look like this (Food -10 Water +10)
The Trade Post in Red Rocket could look like this (Food +5 Water 0)
And in the trade post itself there is a surplus +2 Food
Two problems right off the bat that i'm trying to overcome is the very question I asked, can the buildings talk to each other. Second is if they CAN talk to each other, how to I keep them from mirroring each other.
I also have ideas about security being traded in the form of "Caravan Guards" and taxes being generated by all the trade, but food and water are the basic troubles i'm trying to tackle.