Not sure if anyone posted this yet, but
the TMNT/GB 2 trade is up on Amazon with a slightly more detailed plot synopsis:
They crossed the streams! Turtles and scientists, working together... and they're back for more! The two pop-culture phenomenons join forces again in this sequel to the smash hit to fight a threat more extreme than anything either of them have ever faced.
An enemy from Master Splinter's past is back and hungry for revenge. To get it, he'll summon the most dangerous foe that the Turtles have ever encountered-the Collectors, shape-shifting demons that can pass through dimensions at will and cannot be stopped once they have been summoned to harvest a soul. To save themselves from the clutches of the Collectors, the Turtles team up with the Ghostbusters, the only people who have ever faced them and lived to tell the tale, in a battle that turns into a chase across dimensions both incredible and terrifying.
So the Dark Turtles' true identities are the Collectors, Ghostbusters villains. Nice little melding of the two worlds for a threat rather than just plucking another ghostly baddie from the Turtles' world.