I haven't filled the second wife yet. In a nutshell: Eli and his wife have been married for 10 years but are unable to bear children. He had never abandoned his beliefs from growing up on a compound so he believes their infertility is a sign that he needs to follow the Principle. The wife does not want to share her husband but she wants children loves him enough to follow him if that is the path he chooses to take.
I was thinking the prospective wife could be either a stranger or his as-yet-named aide who has pretty much been his living PDA.