I'm torn about a financial question and am seeking input. Here's the situation:

I'll avoid revealing biases as much as I can. Joe and Joan have been divorced for two years and things have been, for the most part, amicable during that time. Joe and Joan have joint custody 50/50 of their two children.

Joe and Joan have a joint credit card and checking account that are used to pay for all expenses related to their children that are one-time purchases (shoes, school supplies, activity fees). Joe is getting remarried in a few weeks to Mary and wants to add his new wife to those accounts - put her name on the checking account and add her as an authorized user to the credit card. Joe wants to do this for convenience sake as, over the years, surely Mary will have to make purchases when Joe might not be around to hand off a credit card or write a check.

Joan, after some convincing, is okay with Mary being added as an authorized user of the joint credit card because Joe is the primary account holder. However, Joan does not want Mary to be added to the joint checking account because she doesn't want to be financially attached to Mary in a way that could affect her financial profile/history. Joe thinks this reflects a lack of trust on Joan's part and is upset that Joan won't allow Mary to be added. He points out that Joan trusts Mary to drive carpools and to co-parent her children and that Joe in turn trusts Joan's long-time boyfriend to do the same.

What do you think? Is Joan correct to not want to add Mary to the account? Or, is Joe correct?

Thanks for helping me (us) think this through!