Since 2-seater boats were added in 1.9, why not have something similar for land?
Horses will be able to hold 2 players / mobs, (possibly at the cost of a slight speed reduction). Just like in boats, the second player riding will not be able to control, but when the driver dismounts, the second is now in control.
Ways to achieve:
A new, craftable saddle made from two saddles (or maybe one saddle and an extra leather?) to replace the current saddle.
A new slot in the horse's inventory that allows placement of another saddle.
I think it's a fantastic idea, actually. There's been a number of time's I've had to go slowly to keep up with a friend on foot whilst I have a steed, when we could have just ridden together.
I think two folks should be able to share one saddle, but maybe a double saddle could remove some of the speed debuff (not all the way, but a bit), because it's "distributing the load more" or something like that.
Also, nice Zalgo in your PSN and Xbox. H̷̼̳̼ͣ͋͂̍e̠͔̤̍̈́̈́ͫͬ̊̚ ̦͐ͫͨͨ̒̓c̞̜ò̸̘͓͌m̸e͔̜̯͓͍̙̒͛͆̒̽̊ș̨̘͙͕͉͔̞̓͊ͪ.̙̜̻͔̱͓̽