I'm not sure its an actual trade, just a different way to construct a CS meal. At our resort Grab & Go we were able to geta bottled beverage, bakery item and something more like an entree(cheese and fruit plate, PB&J, wrapped sandwich, etc.) for a CS credit. I also think there was a two drink combination as well. We did this to use up our final 2CS credits and I know I had more than two drinks. I had a banana, apple, PB&J, OJ, water and I'm not sure whatelse, but the CM was very knowledgbable as to the combos we could get. I've never read a written policy, but would sugget you check at your resort when you check in.
The only other places I'd think this might apply is the CS bakeries (Main Street in MK and the Boardwalk). I am less familiar with the Epcot places, but perhaps there too, though it seems more of those places are privately owned do they are probably more rigid in their interpretation of a CS meal.
Hope that helps!