I've been working as an indoor skydiving coach for more than 10 years now, and the general way of learning how to fly in a tunnel is by booking time and booking a coach and flying alone together with that coach.
This means you have to pay for the full price for the tunnel time and the full price for the coach, which often turns into close to 600 euros per hour.
And from what I've seen and through my personal experience, flying together with another person, especially for the first 0 to 10 hours, does not harm your learning process significantly and you might lose 10-20% or you could say that you could potentially learn 10-20% quicker if you were alone in the wind tunnel and you had all of the coaches attention.
But if we add it up, then even if it was a 10-20% difference, then by getting double the amount of time, you will learn with a much lower cost by sharing your time with another flyer, especially for the first 0 to 10 hours, it will definitely be economically beneficial to share the time in the tunnel.
Later, things start getting a little bit more complicated still, sharing is a good way to get more tunnel time and also to learn other things like awareness and positioning. But the biggest thing, of course, is the economical benefits and the fact that you will end up flying a lot more tunnel time if you share more.
I've been coaching two people in the tunnel at the same time for several years, and have found a way to make this work. You can take a look at some of the videos (scroll down) from me coaching people two way if you would like to learn how to fly tunnel at the lowest cost possible. I highly recommend you look into this way of doing it. And if you have any questions, feel free to contact me.