In my question What does "do" mean in "What do I do"?, there are 2 answers that are helpful. But when I accept the other one the check mark goes away. Can I only accept one? I want to accept both.
You can only accept one answer, which is the one that helped you more than the other ones. This is by design.
If there are more than one answer that helped you, you can up-vote the ones that helped you, and accept the one that helped you more than the others; you could also assign a bounty to the other answers.
Even if it were possible to accept two answers, there would still be the problem when the answers that helped you are three; if it were possible to accept three answers, there would still be the problem when the answers that helped you are four.
IMO, accepting more than one answer would give to accepting an answer less importance than it actually has. In the case it would be possible to accept all the answers, accepting answers would completely lose its meaning. (What would accepting answers mean when all the given answers are marked as accepted?)
It seems to me that an accepted answer ought to be one that totally solves the problem, and is no worse than other answers. This is much more objective than "best" or "most helpful". I have no problem with there being thirty accepted answers on some question, but only one on most questions. I would implement this feature request.