First off... synergy isn't always about types. Think about dat double-dragon core. They just rip through by eliminating each other's counters.
Synergy could also be about stats in terms of something like SkarmBliss.
![Keldeo](https://img.pokemondb.net/sprites/black-white/anim/normal/keldeo.gif)
![Terrakion](https://img.pokemondb.net/sprites/black-white/anim/normal/terrakion.gif)
Anyhow... Keldeo and Terrakion. Interesting.
Well, first off I see two devastating sweepers in their own right. With equal stats on each side of the spectrum in terms of attacking, they both are pretty awesome by themselves.
Dual-fighting core. With Water and Rock STAB. This actually looks decently viable.
Keldeo and Terrakion also serve as an epic anti-weather core, with Keldeo dealing with rain, Terrakion dealing with sand, and both easily countering sun.
Overall... not the best core, but certainly decent.