So I am planning on seeing my first double with two lovely ladies.  I am wondering should I use two envelopes or one?  If two, do I mark them separately?
Thanks for your input.

asking the two lovely ladies?     Check out the thread about doubles, nice information provided---main point was to see two ladies who are comfortable with each other.   Saying that, I'm sure they wouldn't mind you asking about the envelope.      

That's after you are verified!!!  

Good luck, hope we see a review.

In many cases, the double session is arranged and organized by one of the providers. Usually a client contacts a provider, asks if she has a doubles partner, and makes the arrangements through that first provider.  In that case, I think it appropriate to put the fee in one envelope, and give it (or indicate it) to the arranging provider. That way the girls can work out the split between themselves.  Often, the girl who does all the contact and organizing work gets a bit more than the second partner, and both providers may be uncomfortable sharing that split information with a client.

When you fill the single envelop, make sure it is pretty easy for the women to divide the money.  For example always include a variety of bill sizes that total your fee.  For example, if you are paying $400, don't just put 4 $100 bills, but include some breakdown, like a few twenties and tens.  If the women aren't getting an equal share, it makes it easier for them to each take their share.

That way, there would be no misunderstanding as to who gets what.  If the rates are different, no one gets mad that one girl made more than the other.  And, if you slip one or the other a tip, the other doesn't know and won't feel bad if she didn't get a tip or didn't get as much of a tip.

I do doubles with a FS provider, and even though I'm arranging it, she obviously gets more.  However, I'm working on doing doubles with another provider who the client has also seen individually, and in that case, I asked him to work out her pay with her and to just pay me my rates separately.  

So, I have a different answer for each situation.  Therefore, just ask the provider.