I do not quite see why there is the weird restriction of only one bounty at a time. In fact there have been times when I've had a bounty out and then wanted to place another on a question. If bounties are suppose to be encouraged, then why this random limitation?
They probably want to limit the amount of bounties available. I mean, you don't want too many questions in the bounty otherwise they won't all get answered and therefore that means completely wasted reputation.