A Lasting Power of Attorney is only effective when you lose your mental capacity, and it is a creation of the Mental Capacity Act to allow an adult with the requisite mental capacity to chose his Donee or Attorney to make certain decisions upon him losing his mental capacity.
A Power of Attorney only allows your Donee or Attorney to act within the powers given by you thereunder to act on your behalf when you still have your mental capacity. When you lose your mental capacity or die, the powers granted under the Power of Attorney shall be invalid or ineffective.
Usually, it is a matter of trust that the Donor places on each Donee to act in his or her best interest. If the Donor is comfortable with any Donee making decisions on his or her behalf, then he or she should select the mode “jointly and severally”.
It is ultimately a matter of choice for the Donor to decide.