Remember something else that bugged me. That bit in the Engine Room about people not being able to touch each other or time.. reasserts itself? And they fast forward to zombie status?
The zombies really did feel like a contrived means of having a Monster of the Week to run away from.
The Doctor simply recognized the two of them being fused together. So if they didn't touch, they wouldn't fuse.
That would result in a paradox which he has some recent positive experience with.