Bookings with two therapists arrive in two shapes, they are routinely confused with one another, and the difference matters because it changes what the room has to provide.
A four-hand session is two therapists working on one person, simultaneously, in coordination. A couples booking is two therapists and two guests, each person worked on at the same time in the same room. The first is about what four hands can do that two cannot; the second is about two people spending an hour in parallel rather than in turn. They need the same floor space but not the same money, and past that they have very little in common.
What the room has to hold
Two lanes rather than one. That is the whole requirement and it is more demanding than it sounds in a city whose hotel stock includes a great many narrow doubles.
A single therapist needs clear floor down one long side of the bed, which almost every room in Amsterdam has. Two need clear floor on both sides, and in a couples booking they need enough total space for four adults to move without collision. The junior suites and the larger rooms out towards the ring hold this easily. A canal-house double where the bed has been fitted against the walls on three sides does not, and the honest recommendation there is a longer single-therapist booking rather than a cramped pair.
This is why a two-therapist booking always comes with a question about your room. Nothing hangs on the answer except which format is sensible, and asking twenty minutes early is better than two people discovering a floor plan on arrival.
Arriving as two
Usually together, which is simpler for everyone. Two visitors arriving at the same moment need one call upstairs, one trip down to the lift if the building asks for it, and one knock.
So they come in as two, follow whatever the hotel asks of visitors, and go up in the same lift. If the building runs card-locked lifts, that is arranged as it would be for one arrival, and you come down once rather than twice.
The price, which does not have a trick in it
For one guest, two therapists are three hundred and sixty an hour at any length, which is simply two rates running in parallel: seven hundred and twenty for two hours, one thousand and eighty for three. A couple with a therapist each pays twice that, seven hundred and twenty for the hour and one thousand four hundred and forty for two, and in neither shape is anything added for the night, the weekend or the journey. Both columns are printed on every therapist's profile.
The eight-hour night has a fixed figure in both shapes as well: two thousand eight hundred and eighty for one guest with two therapists, and five thousand seven hundred and sixty for a couple. Half of either is sent ahead once the office accepts, as with every booking that brings two therapists to the door.
Which of the two you actually want
If you are on your own, the question does not arise and the answer is four hands, which is the one format that genuinely cannot be approximated by booking longer. If there are two of you, the couples format is almost always the right shape, and the pattern in the bookings is that it happens on the second evening of a stay rather than the first, once the city has been absorbed and the room has stopped being new.
A last practical note: tell both people. The best couples bookings are the ones with both names on the message, and ten minutes of notice converts a surprise into something both people walked into willingly, which is what the format needs in order to work at all.