If two or more attorneys-in-fact are originally appointed and one dies, resigns, or is unable to serve, a successor attorney-in-fact named in a power of attorney executed in conformity with section 523.23 or a form prepared under section 523.231 replaces the attorney-in-fact who dies, resigns, or is unable to serve. If the original attorneys-in-fact were required to act jointly, the attorneys-in-fact acting at any time must act jointly. If the original attorneys-in-fact were allowed to act individually, the attorneys-in-fact acting at any time may act individually. If attorneys-in-fact acting at any time are required to act jointly, and there is only one remaining attorney-in-fact because of the death, resignation, or inability to serve of all other original and successor attorneys-in-fact, the remaining attorney-in-fact may act alone.