Does my ex wife need permission from her separated husband for me to see my children he adopted? How?

If he adopted the children he absolutely has rights in regard to the children.

You have no visitation right except through your X and her X2B.

Not enough information to give an adequate answer.  If, however, you are not a danger to the children, and depending if you have always been their lives, you will have a better chance of visiting with them. 

In your circumstances it would appear to me that it would be possible for you to have a relationship with your children during your ex wife's parenting time with them. Given the fact that you have terminated your rights, they are indeed terminated, and therefore you would be treated in the law like any other adult person. It is possible for her second ex-husband, who adopted children, to object to using the children but it is likely the court would not enforce that unless there is a very good reason. In the meantime, during her parenting time there should be no reason you should not be able to visit your biological children, unless there is a court order in effect prohibiting.

Legally, he is their father not you. He definitely has a say in what the children are told.