I think it's acceptable to add more than one answer to a question if ... you have more than one answer to a question.
If they're both helpful answers, why not? This doesn't seem any different than answering a question once, describing two completely different methods. That, and the SO engine allows it. If it were undesirable to do this, it would be disallowed, right?
Or, if you answer a question, and then some time lateryou happen upon a better way to do it, then it would be reasonable to add another answer. It would make interpretation of the upvotes less confusing (if anyone cares) because it would be clear that the upvotes were for the original answer, and not the original answer PLUS the answer you added later.