I don't know about credit unions, but when we added hubby to my US bank account it was as simple as them filling out one page of paperwork, including his SS number, and both of us signing it.
As far as taxes, I would guess you could either file married filing separately, or married filing jointly. The first year we were married and he wasn't eligible to work in the States, we figured it both ways to see which gave the best refund. We used H&R Block online, so it was free to try both ways, since you only pay once you are definitely filing. There weren't any bank account issues to come up with taxes. With both names on the account and both names on the tax form, it was simple.
We didn't come across any other issues for having him on the account. Others may have, and could mention them, but we were fine.
Also, someone else with more tax knowledge might be able to answer the tax question better than I did, but that was our experience. We didn't have any issues at all the whole time he was over there. Of course, for us it was no kids, we rented, and we didn't have any stocks, out of state purchases or any other deductions or complicated things to file.