I just watched the video of the end scene on YouTube, and it made things a bit clearer. In it, Ganon's triforce mark on his hand fades out while he's standing there. Then, after he speaks to Link and Zelda, as he grows weaker, it goes to Zant, who is blurry, along with the background. Their poses imply they're looking at each other, though Zant clearly isn't there, from the other views. Zant seems to look at Ganon before his neck breaks. Instantly, Ganon's eyes turn white, and he dies. It does seem to suggest that Zant dying results in the death of Ganon, somehow. The pact idea does seem to mostly make sense now, considering Ganon lost the Triforce, so he was vulnerable to Zant's powers, and considering the order of how things progressed.
I don't think it was Zant causing Ganon's death though. I think the two of them were connected by the Triforce's power and the pact. I think Zant was being kept alive by its power through Ganon, considering what Link and Midna did to him. Once Ganon lost the Triforce due to the Master Sword's wound, Zant was essentially taken off of life support. Once he died, the pact was broken between them, since Ganon promised immortality. The instant it was broken, Ganon's life was forfeit.
Anything missing there?