Leaving aside the Inhuman Toughness for a moment, I can't see how even two perfectly evenly-matched pure mortals could go more than a couple of exchanges without at least inflicting stress. Even if they roll the same result every time, the result will be a 0-shift hit, inflicting 2 stress due to the weapon. And odds are decent that there will be the occasional 1- or 2-shift hit (or much more rarely, up to an 8-shift "critical" hit) resulting in more stress.
So I'm guessing that one of two things are occuring:
1) You're adding the weapon strength to both attack rolls. If so, you shouldn't be; compare the attack rolls to determine if there's a hit, then if there was, add the difference between the attack rolls and the weapon strength to determine stress.
2) Both combatants were armored, and decently so (ie, greater armor than weapon strength). In this case, well, yes you could end up with the result you got. But then again, if two knights in full plate armor start whacking on each other with short swords, it may well take a while for them to wear each other down (by getting lucky hits).
Back to the Inhuman Toughness situation, I have to agree with the responses. Inhuman Toughness means that you are ... well, tough to a degree that is clearly inhuman. And that *should* give a significant edge in an otherwise equal fight. Even so, every hit against the the tough guy is going to inflict at least 1 stress (0 shifts + 2 weapon - 1 armor), so he's going to get worn down at some point due to the stress boxes rolling up. Though it might take a while, and probably the guy without the Toughness will get worn down much sooner.