RESOLVED.
The solution was delete everything from the first 1095-A info I had already entered under the ACA section, and then start the ACA section over from scratch. Only then was I allowed to add a second 1095-A form. Somewhere along the line it looks like I triggered a bug in the software. One has to wonder what other bugginess exists in this software, and whether it affects calculations... I did detailed screen captures showing the problem and resolution, so I can refer to that as a warning in later years.
