Connecting two batteries is not possible without special balancing circuitry AFAIK, you can of course replace the battery by something bigger.
As for how long the battery will last, the nominal duration can be calculated by taking the battery capacity divided by the current draw. Assuming that your leds draw 20mA each and your scout can be in sleep mode nearly all of the time (with power draw in uA range), your total draw would be about 40mA. Using a 550mAh battery, that would give you 550/40 = 13.75 hours of runtime.
Note that this example assumes the microcontroller draw is negligable, which isn't really possible yet. At best, you can do rough sleep scheduling, depending on the response time you need and be asleep most of the time. When not sleeping the microcontroller and transceiver take about 10-20 mA.
Does that help answer your question?