It makes IHO and Cyrodilic Horses play nice enough.
The commands all do what they are supposed to.
All the relevant horses have also been set to Protected and given unique names, that of course can be changed through the magic of IHO.

The main caveat is that because of how the horse models are made (the saddles seem to be a permanent fixture on the model), it is not possible to use the various horse armours and saddle variants with the Cyrodiil horses. Game shouldn't crash or anything, but there will be significant clipping. My own skills are currently insufficient to try and find a solution for this, but I might look into it at some point. 

And all that said, I'm far from an expert on this. I can follow simple instructions like the ones provided on the IHO mod page, but there might still be some odd rookie mistakes in there.
Please keep that in mind when installing.

To install, simply put the .esp into your load order using your mod manager of choice, and make sure it's below both IHO and Cyrodilic Horses in your load order.

The requirements are Immersive Horses and Cyrodilic Horses, as well as their respective other requirements.

.:All credits go to Mihail and Sevencardz:.