co-mama
n.— «There are other words that may be more appropriate to describe the relationship between a first wife and a second wife. Co-mama is a good one, but some moms who are not ready to share the title may find that word choice offensive.» —“Here’s one way to introduce your ex’s current spouse” by Jann Blackstone-Ford, Sharyl Jupe Mercury News (San Jose, California) June 15, 2005. (source: Double-Tongued Dictionary)