Shax said:
Here's a pretty good one that, as far as I know, hasn't been used yet:
Two brothers we are, great burdens we bear, all day we are bitterly pressed; Yet this I will say - we are full all the day, and empty when we go to rest. What are we?
I was really tempted to say the sun and the moon, but it wouldn't make any sense at all .. After a good amount of pondering, though, I'd like to submit my answer.
It must be either shoes, as they come in pairs (at least normally ^^), and are mostly used during daytime, or it could be your bladder and stomach ... But that's not as - decent .. is it?