Just make sure you keep it as varied as possible and it will be fine😊😊 If you do too much of one thing you might end up with a sour horse
My trainer recommended me to do it, and I would if I had had the time, but unfortunately school takes up a lot of my day.. But yeah, I don't see any problems of doing it - as long as your horse is fit enough!
For example riding a little trip in the woods in the morning, and then jumping later on the day would do nothing but good :)
In addition to that, I think the horse would get a lot happier, considered how much it would be able to move (and get used) in a day 😊
I used to do this when my horse was younger. As long as they are fit enough they should be fine but I wouldn't do two hard schooling sessions a day. I used to school for one and then go for a hack for the second ride.
If your horse is in good enough shape it's no problem! Just make sure he/she recover well between. I recommend to buy a pulse clock, then it's easy see if you train your horse too hard.
Another benefit is if you compete. Many competition horses go two classes a day at shows. So if you sometimes school it two times a day it would not be that big physical change.