Additive property of taste
I would classify the neurological phenomenon of "taste" or "tastyness" as an emergent property (1), and therefore synergistic (i.e. not adequately explained simply by additive effects). For example, when something "tastes like chicken" that's a synergistic sensory pattern in your brain involving the specific ratios of the 5 basic tastes(2) on your tongue, along with your sense of smell, and the sensation of the food's texture in your mouth (is it moist, dry, viscus, etc). This helps explain why simply combining two tasty flavors (coffee+tea) does not necessarily yield something twice as tasty, and also why foods which might not be tasty on their own, can be tasty in the right combination (e.g. potatoes+onions+salt+oil+heat makes tasty hashbrowns, but none of those foods are especially tasty on their own). 

Taste preference (tastyness) also involves a certain amount of neurological learning, which is partially why often people don't like the taste of something like coffee when they are young, but learn to enjoy it as they find it elicits a response from the reward centers of the brain. 

In summary, "tastyness" is most likely an emergent and synergistic property of a system of complex sensory and neurological interactions, and not simply an additive.