I actually split a lease with my mother!
We both started as adults (I was in my early 20s, she . . . a bit older ;)). We took lessons together in a school-horse weekly lesson program for a year or two, and I did a quarter lease at that time to get more saddle time.
About 2 years ago, we found a trainer who was a great fit for us, and she had a horse come back from a lease situation who she didn’t ever want to let out of her sight again. She lets us split the lease fee on this mare, and it works great for both of us!
I will say, the horse definitely knows who is riding, and my mom is definitely the softie of the two of us, so sometimes B will test her limits, and try to get away with being a slow-poke, or wiggling around at the walk, but as my mother gains more and more skills, she’s also getting much more consistently good rides with B!
But I think that is a lot about the mare’s personality, she is the type to always check what she can get away with. So if your horse is one that needs a really confident leader, it may learn it can “get away” with more from your daughter than from you, but I wouldn’t anticipate that being an issue, and it’s certainly not an insurmountable one.
We don’t really jump her alone, and my mother doesn’t jump yet, so I don’t think it’s any harder for B to have two riders.
If we are riding at the same time (or on the same day) we’ll make sure to let the 2nd rider know what was worked on in the first ride, so B doesn’t get too much drilling on the same thing with both of us.
I will say, it might be different if I was younger, but I love sharing a horse with my mom! We don’t live together, so it’s nice to get to see her a lot more than if I was just coming out for visits on the weekends, and we both drive so frequently one of us will get to the barn tack and groom, have a ride, and hand a warmed-up horse off to the 2nd rider. Then head home, leaving B to get put up by the other person. It’s really nice to not have to do all the work surrounding a ride :D.
One piece of advice though, and the only conflict that came up when we started sharing a lease is that I was used to coming out to the barn and doing my lease rides alone, so I thought of it as a private time to decompress/be with horses/not interact with people, but my mom wanted to spend time with me, and would often show up to the barn a lot earlier than what she’d told me so we could chat before riding. It took awhile for us to work out a good system where we can both basically say “don’t show up early” or “leave when you are done with your ride” so we can still get some solo horse-time when needed.
With a 12 year old, I’d imagine there may be a time in the future that she sometimes wants to be the sole person in charge during a grooming session, or just needs a break from Mom. I only say that because I have always gotten along great with my mother, but I remember there being times when my hormones were raging and I just needed to be alone, especially with animals, so figuring out how to do that over the next few years is probably something to have in mind. I think as long as your daughter gets some space to feel like it is also her horse, this is a great idea!
It’s something I love sharing with my mom, and I know a lot of riders my age are really envious of the journey we have been going on together over the past few years. I hope it works for you and your daughter as well!