The Collaborative International Dictionary of English v.0.48:

Coexecutor \Co`ex*ec"u*tor\, n.
   A joint executor.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

CO-EXECUTOR. One who is executor of a will in company with another. In
general each co-executor has the full power over the personal estate of the
testator, that all the executors have jointly. Vide Joint Executors. But one
cannot bring suit without joining with the others.