A healthcare proxy is a document that delegates the power to make medical decisions in your behalf to someone you designate. This document limit your delegate to making healthcare decisions, only. Signing a healthcare proxy does not take away your right to make decisions for yourself. So long as you have the capacity to do so, you can revoke this documents, or execute a new one. The point is not to divest yourself of any authority or responsibility, but to make sure your wishes and affairs are carried out, if something happens to you, by someone you trust.
If, however, you want to give different people different degrees of authority—e.g., give your child access to records but leave healthcare decisions only to your spouse—you may want to execute separate healthcare proxies. This can be helpful if you simply want family members to be able to communicate with medical providers, e.g., with respect to an upcoming medical procedure.