snip test one at a time to determine which one is the 1600 roll. So, take one roll, cut off about 10 inches, run it at 800 or 1600 and see if it's the 1600 film. If not, put it aside to develop normally at 400. The worst thing is that will happen is that you'll spend more time doing it and the "snip test" will give you blown out highlights in the test if it's the 400 developed at 800-1600. If you hit the 1600 roll right off that bat, you'll lose one frame rather than under-developing the entire roll. Two stops over exposed might be usable, but it will depend on how you shot it. Assuming your exposure is accurate, you highlights will shift 2 zones, which may or may not be usable depending on what you shot. This is why I would probably develop at 800. Remember, your shadow details develop in first and the highlights come in later. So shortening your time by developing at 800 will give you adequate shadow detail if that's what you metered for, but might give you weaker highlights, which will require more contrast. Either way, this is probably your best solution for minimizing a disaster.