Some-dude, the problem is with the batteries. Even with a single XM-L light but driven at full power, you might have an issue if it has low-voltage protection. Ordering another light probably brings you back to square one.
You can order the XTAR 2600 batteries, they are supposed to have Sanyo cells inside. Very good and safe stuff. BTW for the light, try not to gun for over 10 mins, the LEDs can take it, but not the driver. If it's very cold in your country (below 5 deg C + slight breeze) you can try for 20 mins.