Just a few things:

The brightness of the IR transmitter in the N900 is what it is. The best you can do is select the 1/2 duty cycle. You can't make it brighter with software, nor would you want to if you could.

As for the bits about transistors: The case sixwheeledbeast is making is valid for some applications, but not this one. Part of the reason being, the system is controlling the duty cycle on the led, so the chances of either heating up enough to cause this effect is minimal.

As for why it's using two, it is probably to control current. When one is on, there are only 2 resistors (equaling 110 miliohm), when both are on, there are 4 (equaling 55 miliohm). And yes, the calculations involved are simple enough to determine that most leds would have no problem handling that.