First of all, this is not game-breaking by any means. It just allows you do a bunch of damage / (healing) with double Xuen up.
So how did this happen?
We pulled 4th boss in Heart of Fear and the tank reset the boss by dragging him to the ramp that leads to the platform. Before he reset the boss I and every other monk popped our Xuen. When the boss went away my cooldown was reset and I still had Xuen to tag along with me. We just pulled right after the boss respawned and we popped our Xuens once again. Now I had 2 of them up and went up to alot (80k+) HPS from eminence without doing anything else. I think we could just repeat this one more time for triple Xuen but I did not check. This should work for every boss on LFR but this fight being AoE is the greatest benefit for the 30 secs Xuen x2 is up.
Screenie below:
imgur: the simple image sharer