When T8s fail, usually only that bulb will go bad, if you see 2 T8s out, its almost always a bad ballast.
T12s on the other hand can have a single lamp go out or both lamps. I think it depends on how the lamp fails internally. Also, if you remove a T12 from a 2 lamp ballast, it will shut off the other light, a T8 ballast will power a single lamp with no problem.
I think you are correct that this is more to do with electronic vs magnetic ballasts.
