What to wear for a couples massage is one of those questions people feel faintly silly asking, which is exactly why it turns up in so many booking messages as a postscript. The short answer is less than you think, and nothing either of you needs to buy. The longer answer depends on the format you have chosen, on how the two of you feel about undressing near each other with other people present, and on the plain fact that a hotel room has no changing cubicle.
Here is how it really works when two therapists come to your room.
Before the knock
Wear whatever you like to open the door. Most couples are in the hotel robes, or in something loose they would happily lounge around the room in, and that is ideal. Jeans and a belt are the one thing worth changing out of beforehand, because the first few minutes after the pair arrive are spent talking and arranging the room, and it is nicer to spend them comfortable than wondering at what point to step out of your trousers.
Take off watches, any rings that come off easily, and anything that dangles. Tie long hair back or keep a clip nearby. If one of you wears contact lenses and tends to doze during a session, take them out first. A quick shower before the pair arrive is a kindness to everyone and helps the oil glide, but it is not a condition of anything.
What to wear for a couples massage once it begins
For oil work, which covers most of what couples book, the practical answer is as little as each of you is comfortable with. Oil and fabric do not get along, so most people undress fully and lie under a towel, which is moved as the work moves so that only the area being massaged is uncovered at any moment. Keeping underwear on is completely fine. Say so, and the therapists work around it without comment. Nobody in the room will suggest that one choice is more correct than the other.
The two of you do not have to make the same choice. One can keep underwear on while the other does not, and that difference is as ordinary as asking for different pressure. It is worth deciding separately rather than one of you quietly following the other's lead, because what feels relaxed to one person can feel exposed to another, and that feeling tends to last the whole hour.
For the wetter and closer formats the picture shifts a little. Nuru happens on a waterproof sheet with warm gel, and nobody keeps clothing on for it, because the gel ruins fabric. Sensual massage and tantra are usually received undressed as well. If that goes further than one of you wants, pick a different format for that person. It is a far better solution than half-committing to something that does not suit you.
Undressing with two therapists in the room
This is the part couples secretly worry about, and it is handled with more care than they expect. Once the room is ready, the therapists give the two of you a minute or two of privacy, stepping into the corridor or the bathroom while you undress and settle under the towels. Nobody watches anyone climb onto a surface. At the end the same thing happens in reverse, which is one more reason to keep a robe within reach.
Being undressed in front of your partner is usually the easy part. Being undressed in front of your partner and two people you met five minutes ago can feel odd for roughly the first three minutes. After that it stops registering, because the lights are low, the towels are doing their job, and each of you is absorbed in your own session. Couples who fretted about this beforehand almost never bring it up afterwards.
What the therapists wear
They arrive dressed like any visitor to a hotel guest, discreet and unremarkable in the lobby. What they wear while working depends on the format you have booked. If either of you would like to know in advance, ask in the booking message, and you will get a plain answer rather than a coy one.
After the session
Keep the robes or a couple of large towels near the bed. Oil leaves skin warm and a little slick, and most couples would rather shower after the pair have gone than straight away. If you are staying in, which nearly everyone does, pyjamas or nothing at all is the natural end of the evening. If one of you has to get dressed again for some reason, a late drink or an early flight, allow an extra fifteen minutes and a proper shower first, or the oil will find its way into your clothes.
If any of this is still a question, put it in the message when you book. It takes one line, and the answer arrives with the confirmation about ten minutes later. The questions page covers most of what else couples wonder about before a first session.