I'm still pretty new, have only met a couple of providers. I've been thinking about setting a date to see two providers at the same time. While I know that it will ultimately depend on the individuals in question, I'm wondering what some of the little and big details that a newbie might catch a newbie off guard? I'm talking unspoken rules, unexpected problems that could arise, etc? Thoughts?

you need to ensure that both the girls have awesome chemistry. Its all about chemistry. I only had one threesome in my hobbying career. It was with 2 agency girls, and the performance was very lackluster. I didn't research them properly. They were great individually, but with together they were just mediocre.

The best way to do research is check for ADs which advertize two providers working together.  Typically, they are a safe bet.

Good luck...

I agree with Curly, chemistry between providers is key. Ive had a few 3 somes in my time, 2 where great, one sucked. The one that sucked was with two providers. The other two where in my personal life. IT was obvious the 2providers wdre not into each other at all. They certaintly werent into eating each other cause they didnt do it. Barely kissed. Dont get me wrong, your cock alternating bdtween 2 hot wet mouths is always good, but just not an all holes barred sexfest.
Id say take your time searching. Watch the ads for a couple months. Look for 2 girls that always work and travel together. Chances are theyre bi and possibly in a relafionship together. Then you'll prob get a session where the 2.providers really get into it with each other and you. Makes for a much better experiance.

It wouldn't hurt to limber up your muscles for ten to fifteen minutes before also.

Have fun.