Armin will not be going for the cornucopia, but he would have told Eren to get stuff for the both of them and he'll meet up with him somewhere later, and try to keep track of him in the bedlam of the Cornucopia. So he'd be able to see him die should that happen :|b

He's going to try to stick to Eren as best he can, because they'll both live longer that way. Armin will be assuming everything around them is going to try to kill them so I'm sure he'll spend half the time dragging Eren away from certain death while being a paranoid little fuck. So this should be healthy for both of them.

Edit: Oh yeah. What are your thoughts on them getting stuck inside (or maybe on separate sides of) the Compound?