isnt the idea you have rider and horse.... otherwise why not just a nice (but less expensive doll? dont think you have to divide strictly equally unless dd is of age to notice any huge disparity and as long as dss are really too little to care!
Does she have a "Baby Born" doll? If so you can get a horse riding outfit to fit her. A lot cheaper than the other doll! (we actually bought DD a Baby born doll at a boot fair for 99p! DD loves it just the same!