In my new horse I used the notifies and no mixing for animations. I tried to use animation groups with the same name in the animtree of the horse and rider, but seems that the sync only works in the same actor.
I have used a notify in the horse anim, at the start of animation. This notify calls a horse function that does this:
Use worldimeseconds to the function can’t be called twice. Probably because the anims groups calls each one when mixed. Using timeseconds are called one for each anim cycle.
I took the horse velocity and divide it by a number, a factor. In this way I have a the rate for the rider anim. I had to test a lot of numbers until find the correct values for each horse animation.
Play the animation in the rider, using these rate value, and the override bool set to true. If the rate is accurate, the animation just begins when the last cycle are finished. As the horse velocity can vary each tick, in this way you update the rate for the rider in each animation cycle.
The result is no perfect, anims are late sometimes when the horse change the run anims, but the sync are good the most of times.