If you are making arrangements with one lady who will bring the second, then one envelope is how you would handle that.  

Now, if you are contacting them separately, make sure they are each aware of what you are planning.  Surprises are not something they want.  

About the donation in total, from what I've experienced that donation would cover them both if you are making arrangements with just one of them.  

The thing about duos is they can be very fun as long as you've spent time with one of them.  Doing a duo with an unknown can lead to disappointments.  I've done a few and I've spent time with each of the ladies individually, so everyone knows one another and it was significantly more satisfying.  There is another thread about duos that was done earlier, just scroll down on the list of posts.  

... and got clarity on the donations beforehand.  (I only do doubles when I know at least one of the ladies beforehand, so they're comfortable discussing donations up front.)  That's important because I've heard of providers who believe that for doubles, each provider should receive the same donation.  IMO each provider should receive whatever her usual donation is for that duration, plus an outcall or travel charge if appropriate, unless there's a specific rate for doubles.  That approach has always worked for my doubles sessions.

I always suggest that you have a provider you already know well, and who knows you well pick the third partner as that was you can be sure that she is simpatico with that gal and that she'll select someone that you'll be happy with.

In my first doubles, I neatly laid out all the cash on the bathroom counter in two sections side-by-side for each ladyr and had two envelopes. They didn't take the envelopes, just the cash.

Regarding the total donation, it's best to ask for confirmation once you're already screened and they know you're serious.

This is a very good question and problems can certainly arise...

I agree that one envelope should be given to the provider who booked the appointment, however, I think the tip should be given separately to each provider.

 The problem has been in the past when the ladies have two different rates. The guy divides the total amount into two, and then the ladies get two equal amounts. Then the lady with the higher rate has to tell the one with the lower rate that she wasn't getting a tip that she actually has some of the other providers donation money in her envelope.  

 Another problem that can arise, is that the provider who booked the appointment might keep the entire tip if it's all in one envelope instead of splitting it with the other person.  

 I would always split a tip 50-50 with somebody, but not everybody does.  

I hope this helps.

Great question.

I have had guys put in one envelope but it is easier to just put it in separate envelopes.
Also I would just check to make sure that the rate on the profile is correct...you could say something like I saw your duo profile with you and xxx the rate mentioned is that for both or for each of you?
I just did a duo recently the provider and I have a duo profile already set up here but the rate had 700 but that was for a 2hr appt and it was 700 each.We saw a guy for an 1hr duo appt and he left 700 he was 100 short which he did give us.But it was quite awkward to have that conversation  he said he was going off of the duo profile that said 700 I told him that was each and for a 2r appt he booked a 1hr appt and we are each 400/hr