Yes, more than one person can be named as agent under a POA. This is not generally a good idea. It also sounds like your parents may have capacity issues, at this point. If they have capacity, they can change the POA. If not, the only way to do this would be to go through the probate court.
***Please be sure to mark if you find the answer "helpful" or a "best" answer. Thank you! I hope this helps. ***************************************** LEGAL DISCLAIMER I am licensed to practice law in the State of Michigan and have offices in Wayne and Ingham Counties. My practice is focused in the areas of estate planning and probate administration. I am ethically required to state that the above answer does not create an attorney/client relationship. These responses should be considered general legal education and are intended to provide general information about the question asked. Frequently, the question does not include important facts that, if known, could significantly change the answer. Information provided on this site should not be used as a substitute for competent legal advice from a licensed attorney that practices in your state. The law changes frequently and varies from state to state. If I refer to your state's laws, you should not rely on what I say; I just did a quick Internet search and found something that looked relevant that I hoped you would find helpful. You should verify and confirm any information provided with an attorney licensed in your state. I hope you our answer helpful!
I agree with attorney Frederick's answer that if they have capacity they can change the POA. If not you would have to look to a guardianship (conservator). Having more than one agent under a POA serving at one time is a recipe for problems (decision making by committee simply doesn't work very efficiently). It isn't clear to me why there is a problem. If the problem is that one sibling is making all the decisions, having an open discussion is probably the best solution...certainly a better solution than litigation would be. The agent is under a fiduciary duty to act in the best interest of the principal. If the decision to place parents in a nursing home is the source of the problem I would first ask what are the alternatives. If the alternative is leaving mom/dad at home, then I think there needs to be an honest assessment of whether the other siblings are really going to provide assistance in the home. If not, then presumably you have a doctor certification that there is a medical need for nursing home placement and unless someone is going to be a full time caregiver the agent sibling has probably done the best thing possible for the parents, which is placing them in a facility with round the clock professional skilled care.
It is impossible to give specific answers to questions without meeting and fully discussing all of the potential issues that may not be addressed by your question. The answer(s) provided should be considered general information and are not legal advice. Only after a thorough personal consultation could specific legal advice be given. Viewing this information is not intended to create, and does not constitute, an attorney-client relationship. To enter such a relationship you and I would need to consult in person and form a mutually agreeable written contract of engagement. The answer(s) provided in this forum is intended to educate you and point to issues for you to raise in a consultation with a lawyer of your choosing who is appropriately competent in the field of law that your question concerns and who is duly licensed to practice in the jurisdiction where you live and/or where the events giving rise to your question occurred. You should not take any action that might affect your claim(s) without first seeking the professional opinion of a licensed attorney. There are often strict deadlines for filing suit, responding to a suit or making an appeal and you need to personally consult with an attorney to make sure that you understand and meet those deadlines.