Indeed, we can always make two things seem indistinguishable, if we eliminate all of our abilities to distinguish them. The two bodies in your case could still be distinguished with an fmri scan, or similar tool. This might not count as "behavior", but then I never wanted "behavior" to literally mean "hand movements".
I think you could remove that by putting the two people into magical impenetrable boxes and then randomly killing one of them, through some schrodinger's cat-like process. But I wouldn't find that very interesting either. Yes, you can hide information, but it's not just information about consciousness you're hiding, but also about "ability to do arithmetic" and many other things. Now, if you could remove consciousness without removing anything else, that would be very interesting.
OK, so what did you mean by "behaviour" if it includes things you can only discover with an fMRI scan? (Possible "extreme" case: you simply mean that consciousness is something that happens in the physical world and supervenes on arrangements of atoms and fields and whatnot; I don't think many here would disagree with that.)
If the criteria for consciousness include things you can't observe "normally" but need fMRI scans and the like for (for the avoidance of doubt, I agree that they do) then you no longer have any excuse for a...
(This post grew out of an old conversation with Wei Dai.)
Imagine a person sitting in a room, communicating with the outside world through a terminal. Further imagine that the person knows some secret fact (e.g. that the Moon landings were a hoax), but is absolutely committed to never revealing their knowledge of it in any way.
Can you, by observing the input-output behavior of the system, distinguish it from a person who doesn't know the secret, or knows some other secret instead?
Clearly the only reasonable answer is "no, not in general".
Now imagine a person in the same situation, claiming to possess some mental skill that's hard for you to verify (e.g. visualizing four-dimensional objects in their mind's eye). Can you, by observing the input-output behavior, distinguish it from someone who is lying about having the skill, but has a good grasp of four-dimensional math otherwise?
Again, clearly, the only reasonable answer is "not in general".
Now imagine a sealed box that behaves exactly like a human, dutifully saying things like "I'm conscious", "I experience red" and so on. Moreover, you know from trustworthy sources that the box was built by scanning a human brain, and then optimizing the resulting program to use less CPU and memory (preserving the same input-output behavior). Would you be willing to trust that the box is in fact conscious, and has the same internal experiences as the human brain it was created from?
A philosopher believing in computationalism would emphatically say yes. But considering the examples above, I would say I'm not sure! Not at all!