Most adults learn the piano alone: in the evening, in an empty room, often with headphones. That solitude is part of the pleasure. But the piano was never meant to be only a solitary instrument, and some of the happiest moments adults describe to me involve one other person on the bench beside them.
Why playing with someone helps you learn
You cannot stop. Alone, an adult stops at every wrong note and starts again. With a partner, the music keeps going and you must find your way back in. That one habit, keeping going, is among the most valuable a pianist can build. Why you stop every time you make a mistake explains why it matters so much.
You learn to count for real. A metronome is patient. A partner is not. When two people play together, a shared pulse is not optional, and counting stops being an exercise and becomes the thing that holds the music together.
You start to listen. Playing your part while hearing someone else's teaches balance: when to be the melody, when to step back. That is the beginning of real musicianship.
You come back. It is easy to skip practice when nobody is waiting. It is much harder when someone is expecting you on Thursday evening.
Who to play with
A spouse or partner who also plays, or is learning. A friend from work who had lessons as a child. A grown child who still remembers some of their own lessons. A neighbor. For many adults in retirement, a weekly duet is also a weekly appointment with another person, and that companionship is worth as much as the music.
The two players do not need to be at the same level. In fact, some of the best duet music is written for exactly that situation.
Music to start with
Duets with an easy part. Many duet collections are written so that one part is very simple, often in a single five-note position, while the other part fills in the harmony. Anton Diabelli's Melodious Exercises, Op. 149, were written exactly this way, and they are real music, not drills.
Accompaniments in your method book. Many adult courses print an optional second part above or below the beginner's line. If yours does, ask your partner to play it; your simple melody suddenly sounds like a complete piece.
Your own arrangement. One person plays a melody you both love; the other plays the bass note of each chord. That is a duet too, and a good way to start.
How to play together without falling apart
Agree on the tempo before you start, and choose a slow one. Count one full bar out loud together before the first note.
Decide who leads. One person breathes or nods at the start and at any pause. Usually it is the player with the melody.
Decide who pedals. In music for four hands on one piano, the lower part, the secondo, usually pedals, because that player's foot is nearer the pedals and their part needs it most. Agree in advance, and only one of you pedals. When to start using the pedal explains the basics.
Never stop for a mistake. If you get lost, keep counting, find the next downbeat and come back in. Afterwards, practice the spot separately.
Practice your part alone first. Duet time is for playing together, not for learning notes.
Use a metronome together at first. It gives you both something neutral to follow. What the metronome is actually for shows how.
Playing with a partner at a different level
Many duets happen between people at different stages: a spouse who played for years and one who is just starting, or two friends a year apart. It works beautifully with a few agreements.
The stronger player supports. Their job is not to show off, but to make the other part sound good: a steady pulse, a soft accompaniment, patience at the hard spots.
Take turns choosing the music. Both of you should be playing something you want to play.
Do not teach each other during the duet. Corrections between partners, especially married ones, are where duets go wrong. Play it through, then each of you practices your own part. If something keeps going wrong, agree to slow down rather than to explain.
Celebrate the first complete run. The first time you both reach the end together without stopping is a real achievement. Record it.
Sitting together
On one piano, sit far enough apart that your elbows do not collide, and check that both of you have the right height. A long bench works; two chairs of the right height work too. On two instruments, place them where you can see each other's hands.
If you have no partner yet
Play along with a recording of the other part. It teaches many of the same skills. Practicing with a recording explains when it helps and when it hurts. Over the internet, live duets are nearly impossible, because of the small delay in every connection, so recordings are the practical substitute.
Music, shared
Duets are where many adults first feel like musicians instead of students: two people, one sound. When you are ready to go further, read playing for a small audience at home and chamber music with violin and cello. If you are in retirement, see a plan for your 60s, and to begin at the first lesson, start here.