Can you have a power of attorney and a legal guardian?

Yes a person could have both. But guardian if smart, will work very collaboratively with the POA.

I am trying to think of an instance it might be useful, but can't. I do wonder if a guardian could appoint someone in case he/she was unavailable at certain times. I've not heard of it, but I guess it is a possibility if it went through the guardian. Maybe someone will know for sure.