Hey Go, it's strange to me that they would be in the same, exact books, at the same exact place within them, and experiencing the same, exact problems as each other. Anyway, do you ever have them doing any kinds of supplement materials to support their reading ? I like to send my students here : https://www.emusictheory.com/practice.htmlor here :https://www.musictheory.net/To give them somewhere else to practice reading notes.
Twins usually experience the same things,
Karli: Perhaps they would prefer to experience the same things~~
It took me quite a while to understand that there are many students throughout all age groups who are not really interested in learning to read music. They love to play but only if they learn by ear, or they expect that you as a teacher show them every note on the piano. They don't see the point in learning notes. And it took me another while to get that I must not support this attitude. A lot of my efforts in the lessons are dedicated to convincing them that it is necessary to learn note reading to become a serious pianist. Very tedious. And I admit that I haven't found thee solution yet, though it has become slightly better since I got that most of those people that seem to have "difficulties" with the notes aren't dyslexic nor stupid nor immature nor anything like this, they're just reluctant. So it's rather a matter of teaching/learning discipline and concentration. And, of course, patience.
What matters most is that it works.
That sounds like you're trying your best and doing a good job! It's a good idea having them do seperate pieces too. Just one more idea (you might already be doing this), how about giving them each a really short piece for homework every week that they learn by themselves?