Is there an expectation that both provider in a duo appointment get paid the same?

Additional information: they work for the same agency, the provider with the lower rate (posted) was who I initially contacted and I have seen about 6 times already. The difference for 2 hours is $200. Like everyone, I like the best deal but is it worth the possible ill will from a regular?

IN GENERAL, at least with indies, if their individual rates are different, $Low and $High, it is customary to pay each $High = 2 x $High = total (before tips).  
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I think that the same would apply in your agency case but maybe you're concerned that your "regular" (who usually gets $Low) might expect $High from you in the future??  
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I don't know how to figure out that problem in advance but maybe others can chime in. For now, plan on paying 2 x $High for the upcoming duo. ("If I pay you $High for the duo are you going to make me pay you $High or $Low for our future 1-on-1 sessions?" is too direct.)
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In the meantime, you might want to search General for opinions on duos. Many (most?) duos (F - M - F' threesomes) turn out to be a F-M to M-F' tag team events. I.e., you get 1/2 F + 1/2 F' for the duration but you're paying for 1F + 1F'.  If you're expecting a porno scene (lots of F-F' action and F-M-F' action), you might want to do some preliminary research. Does either one have an duo reviews?
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Good luck!

For most duos I’ve done, each provider expected her normal incall rate, plus a travel fee if she had to travel to the session. But I know providers who have special duo rates, ie one amount to be split by the two of them and lower than the sum of their normal rates, because they enjoyed playtime with each other so much.

I have done the duo with the lower price provider but I don’t think it was an issue question I considered before. Both providers are BDSM providers so theatrics/roleplaying is in their wheelhouses. I will probably ask the booker as she has come across as a straight shooter for years.