When I worked at Pepsi, they paid 'rent' for keeping a vending machine somewhere. The rent was based on location, power usage, etc.
I imagine at least a few Pepsi employees might like some of the Coke brands (e.g. Dr. Pepper).
Pepsi wouldn't be caught dead selling Coke products in their machines. Some machines have both, but those are offered by third parties (Canteen/Vending companies) that buy in bulk from Pepsi & Coke and resell.
http://en.wikipedia.org/wiki/Dr._Pepper
Wiki seems to indicate that Coke manufactures it, but only does so in Europe.