Kristen, as everyone has said it's hard to give advice when I have no knowledge of the property. On an established pasture in Georgia it would be no problem to pasture board two horses. The reason is our growing season and weather. However I know in some areas out west the requirement is much larger. As previously said it would help if you contact the county agriculture agent responsible for the area you want to move to. They should be able to advise you how many acres is needing to support a horse in that area.
Be sure to factor in the sheep you want to keep when figuring what the land will support. Also have a boarding agreement that spells out resposibilities. Who provides supplemental feed (hay, grain,etc.), who is resposible for general care (bathing, farrier, worming, vacinations), and who is responsible for emergency vet care. In my boarding contracts I always have a clause where if in my opinion the horse needs vet care and I can not contact the owners I can call in a vet and the owner is resposible for the bill.
Good luck and I hope it works out.