Duo massage is a search phrase with two readings, a pair of guests or a pair of therapists, and the office always asks which one you mean before quoting.
"Duo" is a borrowed word, common in German, Dutch and Scandinavian spa brochures, and visitors carry it with them into English searches. It is short and tidy, which is why people type it. It is also ambiguous, which is why the office never assumes.
For most people who write "duo massage" in the booking thread, the duo is the two of them. They are a couple, or two friends, and they want to be looked after side by side in one room. In the house vocabulary that is a couples massage, and it is normally booked with one therapist for each guest costing EUR 360 per hour between the two therapists.
A smaller group means something else entirely. For them the duo is the therapists, two women working on one guest together, which the house calls four-hand. It costs exactly the same per hour as the couples version, since the fee counts therapists at work, not guests.
The identical rate is the reason confusion rarely costs anyone money. It can still cost an evening, though. A couple who booked four-hand by accident would find two therapists arriving for one of them, and a solo traveller who booked a couples session would find a second table with nobody on it. So the office asks a simple question before it accepts: is the duo the two of you, or is it the therapists?
Now and then "duo" means a pair who want one therapist to share between them, one partner and then the other. That is neither of the readings above and it sits on the single-therapist ladder: two hours at EUR 360, for instance, divided between you however you like.
Whichever duo you are, spell it out when you write. One sentence, such as "two of us, a therapist each, ninety minutes", saves a round of questions and gets the confirmation back sooner. The booking page lists the other details the office will need.