My younger brother is being taken to a nursing hm, My older bro. was my Moms executor

First of all, adult guardianship or conservatorship is a very big and expensive step, requiring court hearing and usually hiring a lawyer. So, before starting down that path, you might want to consider the less complicated and less costly alternatives of an advance health care directive, which is a legal document that would allow you to make decisions about your brother's medical care, and a durable power of attorney for finances, which would allow you to take care of his money matters. Regardless of your brother's physical condition, if he is mentally capable -- in legal parlance "legally competent" -- of understanding what a health care directive and power of attorney document do, then he could discuss these documents with you and sign them, giving them legal force. They would allow you to make decisions for him, including decisions about his care in the nursing home, to the extent he cannot make the decisions himself.

Whether your brother can execute these documents, or instead you must proceed with conservatorship or adult guardianship, there is another issue you raise, which is who should be granted authority to make decisions under these legal arrangements. You ask about "dual guardianship," and since you mention your older brother, I am guessing you mean dual authority with him. Whether under power of attorney or guardianship, it is legally possible to have two people hold legal responsibility at the same time. If neither of you lives near your younger brother's nursing home, and each of you will, at different times, need to oversee your younger brother's care, then dual authority might be a good idea. But you should be aware that dual authority can also cause serious problems. If you and your older brother do not agree about something concerning your younger brother, the dual authority can create a stalemate where neither of you is legally able to provide a final decision about something that might be important to your younger brother's health or welfare. For this reason, most lawyers counsel against dual guardianship or power of attorney authority. Instead, the guardianship or power of attorney can be set up with one of you as the primary authority, with the other as a secondary (or back-up) authority who can act only if the primary authority cannot or chooses not to act.

Finally, whatever route you take -- conservatorship/guardianship or power of attorney -- you will be acting under the laws of South Carolina, where your younger brother will be living. The place where he is a resident, not where you or your older brother live -- governs the legal documents and proceedings that will affect your younger brother.