Yes the Perry WOTR and Agincourt plastic sets use the same plastic horses.
HOWEVER if it's relevant, note that all the metal horses in both these ranges are quite noticeably smaller than the plastic ones. Less height, and often quite skinny looking. They work really well for lighter cavalry or independent commanders, but if mixed in the same unit the size difference is noticeable. You don't want your metal men-at-arms to be on little horses and your plastic mounted archers on great big ones! Or a metal commander leading a unit of plastic heavy cavalry, and he's on a clearly smaller horse. Just a pet peeve of mine!
I intend to remount the plastic light cavalry on smaller horses, probably TAG metal ones or plastic from Wargames Atlantic. For now, I need large horses knights, preferably barded. The Fireforge ones only have two poses and the heads leave something to be desired, though they do match the Perry ones and could use the spare heads from the latter for the former. I have been thinking of attaching thin plasticard, with the aid of a small heat gun, onto the Perry plastic horses, but not sure if it'll work: heat a cut piece, then quickly attach to the body, so it conforms to the shape and then trim after it cools.
The easiest option would've been the new horses from Victrix, but according to their FAQ, they don't sell sprues, as these come pre-packaged from the factory - no idea why it can't be done, as Perrys sell horse sprues.