I had two in a previous life because the company I worked for had reason to dispatch me on assignments in Israel or hostile countries in the Middle East which do not recognize Israel and will deny you entry if their is a Israeli stamp in your passport. Nothing illegal about it if you bring an authorized letter from your company requesting the same to your nearest U.S. Embassy or Consulate.
They will, of course, call to verify and collect the passport fee twice. You are, of course, told to turn in the spare passport once the reason for having two no longer exists.