When a story threatens to spill out

As you all know I've been trying for the past two and a half weeks to finish my story for the DDD anthology (the deadline is Aug. 1). Notice that I said "trying." I started alright, adding about 1.5K words to the story during the first week, but then I hit a (huge) snag, and I've not been able to move past it. You see, the problem is that the external conflict is taking over the internal conflict. Okay, well, that's not it exactly. Hm... It's more difficult to explain that I thought. Here's the deal, I'm trying to create a connection between the DDD story and another one, which I started writing even before DDD one. Besides having the same setting, which is also the common denominator in Viviene's and Cassandra's stories, I wanted Zack, the hero of my other wereanimal story, to appear in Marcus's story (which is for the DDD anthology). You know, have a couple of scenes together so the readers would recognized Zack when they read his story. So far, so good, right? Right. So for the DDD story, I wrote a scene that allowed me to introduce Zack, and then a second one in which Marcus and Zack interact a bit more, showing the relation between these two. Fine. Now here's where the mess started. Zack's story is more action-packed whereas Marcus's story is more internal conflicted. But because of the scenes I already wrote, the external conflict in Zack's story is sort of spilling out into Marcus's story. I want Marcus and Helena's story to have more internal conflict than external conflict. I mean, the external conflict will be there in the form of her ex-husband, but I don't want this story to turn into one of those "action-packed ones," where the hero/heroine depend on something to happen to react. You know, the typical line in which the hero saves the heroine from perilious situations. No, no, no!!! This isn't what the DDD story is about. I want Marcus and Helena's story to be very emotional. I want to make the heroine struggle with her own emotions and believes (what's right and wrong), and therefore, raise and challenge society's moral values. (But it's all an emotional struggle for her. She will have to break with everything she grew up believing in and change.) So, my question to all wise writing gurus out there is: What do you do when the conflict/action of one story threatens to spill into the other? How do you keep the original plot intact without allowing the follow-up to mess it up?