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.
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.