If its an account upgrade, it would be stupid (no offense) to have to go into your account settings periodically to turn the option to own two houses on and off. I would not want an increase in reoccuring charges because not everybody needs or wants two houses and would want to be impacted by that fee. It may be something I would be interested in, but it presents problems of balance.
I could definitely see myself purchasing the upgrade if there were protocols inplace that prevented someone from owning two houses on the same shard. For instance, you can buy the upgrade and you would be eligible to own 1 house per shard, or two houses altogether, 1 on two unique shards. In the event that this did happen, I would also like an added house menu that allowed you to set your primary/secondary status on the houses you own. That way you don't shard hop, place a new house and your primary house falls (it would only affect the one set to secondary status).