The first thing to realize is that unlike some other LEDs, the XML series does not have 2 independent emitters. It only has one emitter, and the MCPCB that you have has brought each terminal out to two pads. This is to make it easier to wire several in parallel.
If you were to run the buckpucks on two independent and isolated power sources, this would probably work okay. But running one battery, one LED, and two buckpucks is not something it's designed for. A quick google search reveals that some people have tried it with poor results. Several said you get nowhere near twice the current (and as a designer of buck regulator circuits, this does not surprise me). One reported very high current for a brief period before the LED failed. I suspect that was due to a wiring mistake, but there's no way to know.
Now, if you chose a part like the XHP50 which actually has independent emitters, this would work fine from a single power source. This part actually has 4 emitters, wired as 2 isolated strings of 2 emitters.