So I got two votes for more life, one for same life, and none for different brightness.

The older light with two batteries, I put them in as you describe "in series", both down the same long tube, buttons (positive?) inwards, can't imagine doing it otherwise. Could it be possible that I've been putting my batteries in wrong?

I guess I could do a rundown test, charge two batteries and see how many hours the 2-battery light gets on high, then charge them back up again and see how many hours the 1-battery light gets on high (measure twice and average?); use the same batteries to eliminate any difference in battery quality