Is it considered unethical/bad etiquette to contact another potential PhD supervisor while waiting for a response from my first choice?
Finding a PhD advisor, like finding a job, is not a serial process. You can—and should—have multiple applications out simultaneously. If you get an offer you find acceptable, then you should take it. 

If you are concerned about missing out on an opportunity with your first-choice advisor if someone else makes you an offer, then you can send a polite email informing the first-choice advisor something like:


  I would really like to work with you, but I am starting to receive offers of admission from other advisors. They would like to have an answer by &lt;date>. While I understand you need time to make a decision, would it be possible for you to give me an answer by then?


If they're serious about having you as a PhD student, they'll respond.