If the OP wants to treat them as *one* lamp, I would seriously consider a small DC-DC converter to kick the 5V (assuming it wants to be entirely powered off the USB) supply up to "whatever" @ 10mA. Note that you could design the supply so that *it* does the intensity control (current mode output that tracks something from the MCU).
I would be more concerned about having enough Icc from the USB port to deliver what the lamps will consume. (high efficiency lamps?)
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Would work, but I'd be concerned with process variations between the LEDs leading to the pair with the lowest forward voltage drop hogging all of the current. Less of a problem (and likely a non-problem) if they're all from the same batch, of course.
A current mirror is probably the best approach for balanced control. Also opens the possibility of controlling the brightness by controlling the current directly (and avoiding flicker or beat frequencies).
-- Rich Webb Norfolk, VA
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