What is the etiquette to follow when
- there are two or more valid solutions to a question
- when a solution is derived from two or more posted solutions
To whom should I award the "answer"?
If they both provided a useful answer, then you can upvote both of them, that is how you can provide some reward to both.
Pick one based on what you liked the best to accept, then you can add a comments to each making it clear why you chose the one you did, but that you liked the alternate as well for whatever reason. As other people find your question they may upvote both answers based on your comments endorsing the answers.
The 15 rep from the accepted answer on a single question really doesn't make that much difference. A good comment thanking the person who provided the answer in some ways can be even more rewarding.
To whoever you want. To the first one posted, to the most funny one, toss a coin, etc.
The decision is yours and there are no particular guidelines in this case.