if you only plan on seeing one of them? I understand some providers might not to spend time communicating with someone if there's no commitment, but is it something that's widely frowned on?

I'd say yes, it is not a good idea.  You would have to tell the ones you aren't seeing them that you won't be coming to visit them.   If you have made only the one appt, and just chatted with the others, you may get the rep as a time-waster, and that's not good.  Once you have seen a number of girls, and I don't know if you have, you can have one regular you could use as a plan B if your primary target is unavailable.  Busy providers don't have time to just chat on the phone or spend time texting back and forth.  In their  business, time is money.

LLAP,
Swim

The only times it's not bad form to take the "shotgun" approach to seeing providers are when you are planning multiple sessions perhaps for a several day trip in which you plan on seeing multiple women, or if you are trying to find a "same day" appointment in which case it's still proper etiquette to let the ladies you contact that you are contacting multiple women as you realize that most of them will be unable to accommodate you.

The proper etiquette if you are looking for an appointment lets say a week from now is to contact the woman you most want to see, wait a respectful length of time, at least 24 hours IMO and if you don't hear back, then go to the second woman on your list. To do otherwise is to risk being labeled a time waster and keep in mind that especially if you are including references, these women talk to each other and often times are friends. It's quite likely that the word will get out about what you are doing leading you to end up on a lot of DNS lists.

So yes, in answer to your question, it IS something that is widely frowned upon.

Well, that makes sense. I wanted to put effort into finding the best match possible (not just who I would prefer, but who she would prefer as a client) and I thought it would make sense on both sides. When I realized the possible complications I did include a disclaimer with my questions that I was talking with other providers. The ones I told haven't responded... hope I haven't soured things too much. Gah.

Right up there with stalkers, abusive clients and guys too dense to wash their own ass, "time wasters" will be right up there at the top of the list. You most certainly don't want to be "that guy"

Just remember this is not Match.com, this is a business for these women. They aren't looking for a connection, just a clean, respectful guy, one who won't push her boundaries or waste her time.

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You might find it useful.

we are still alert for time wasters. While I actually prefer to screen out someone who won't be a good fit, I still don't want to deal with a time waster who has no intent on seeing me. Your email that mentions you are talking to other providers might come across as a bit condescending, too (i.e "I might consider you if ....).  

You probably want to narrow your communication down to someone you really think you want to see by reading her reviews and website. You can perhaps do some backchanneling as well if you still aren't sure you're a good fit. It sounds like you're hesitant to take the plunge, but if you're a nice guy and follow basic etiquette, the worst case (assuming you see someone reputable) is that you will have a good time instead of a mind-blowing time.

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...that have seen the provider you're interested in by PM (or other method) to see if they have any pertinent info to share regarding your questions about her.