I just did a quick test. And this is the sort of thing you can do too.
Just taking 5V from a power supply, and running it through one red LED with a 220R resistor I measured 14.7 mA. This sounds right because the LED had a forward voltage drop of 1.7V, so:
current = (5 - 1.7) / 220 = 15 mA
Now put two LEDs in series and I measure 7.2 mA (about half). And this would be because the current is now:
current = (5 - 1.7 - 1.7) / 220 = 7.2 mA
So each LED is only getting 7 mA rather than 15 mA and is thus less bright. Now you could compensate by halving the resistor.
Personally I couldn't see a heap of difference in the brightness, but I think that is because our eyes to not respond linearly to brightness.