The first thing you should do is make sure the person you want to share your lesson with is about the same standard as you and has more or less the same objectives.
Over the years, I've had a few people come to me and ask if they can share the lesson, and I have always said yes.. Often this was done to share the cost, which for some people is hard to justify on their own. Other times it was done simply for fun - they thought it would be fun to do with somebody and had "less" dedication. I say "less" with inverted commas because in general players who really want to improve have lessons alone, but it's not always true.
Not all coaches will accept shared lessons, so double check with the coach you want to do it with first. If two players arrived for a lesson with me, when I only had one scheduled, it wouldn't bother me at all, but it's better to inform the coach.
The next point to consider is what each player wants. For example, if one player wants to work on their technique and the other player wants to learn some new tactical tips, that might make the lesson less fun because technique often requires stopping and starting. I'm not saying it's impossible for each player to have different goals, I'm just saying that you should discuss it together first and then with the coach.
One Of The Benefits
One of the benefits of having shared coaching is having the coach give you practice drills, both solo and pairs, for you to go away and do. The coach can make sure you understand exactly what you should be doing with each drill and tell you how to progress. Then, after a set agreed time, you can return to the coach and check your progress.
I believe every coach should be giving "homework" to their pupils. Very rarely is simply playing matches enough or effective in improving your squash. Having a training partner is a great way to stay motivated to DO the homework the coach gives you.
Players learn and improve at different rates and in different ways. In the past, over a 6-month period, I had a shared lesson, where one player really improved and the other didn't improve as much. This caused a little frustration in the slower learner. We adapted the lessons as best we could, but eventually they had to stop as the difference grew too great and the slower learner stopped enjoying the lessons.
I still wonder whether I could have done anything different, but I don't remember the specifics enough to really analyse the situation. I also want to mention I sued the word "we" because the pupils should have some say in how lessons are planed and executed.
If the cost of a lesson is too hard to justify or you want a friend alone for fun and support, try shared coaching. As I said, talk about what you both hope to achieve and check with the coach first. Many times it is cheaper than a single-person lesson, but it is also slightly more expensive. For example, a single-person lesson might cost 25 Euros per 45 minutes, but a two-person shared lesson is 30 or 35 Euros.