A duo is a booking with two companions at the same time, who already know each other and have agreed in advance to work as a pair. That last part is the whole thing.
Why the pairing has to be theirs
People often assume a duo is something you assemble by picking two listings you like. It is not. Duos exist because two people have an established working relationship, trust each other, and have decided together how they operate.
Asking two companions who have never met to pair up for your evening will usually be declined, and politely. It puts both of them in a situation they have not agreed to with someone they do not know, which is exactly what nobody in this line of work wants.
If someone offers duos, she will say so, and she will name who with.
What it costs
Expect close to the two individual hourly rates added together, occasionally a little under. You are booking two people’s time and both of them are turning down other work for it.
Duos also tend to carry a longer minimum than a single booking. An hour split three ways is not much of an evening, and most pairs will ask for ninety minutes or two hours as a floor.
Sort the details in advance
More than a single booking, a duo benefits from being specific when you enquire. Say how long, where, and roughly what kind of evening you have in mind. Two people are coordinating diaries, so late changes are twice as disruptive.
Incall is more common for duos, simply because it is easier to host two people than to send two across London. The incall and outcall guide covers the difference.
Booking one
Enquire through whichever of the two you found first and she will coordinate with the other. Do not send separate messages to both, which creates confusion and two half conversations. The booking guide applies as normal.