What are some ways to let both my adult children share being executors of my will? I don't know how to let them share this responsibility?

Write it in your will.

Consult an attorney. Put it in writing. Make a will.

This is a question for an attorney familiar with creating wills. She/he will have insight and wisdom as to whether having 2 executors is advisable. Will probably need to know why you want 2. Hopefully those 2 get along?