If we were to submit this to two different journals, is it plagiarism?
Yes, reusingReusing text from another paper (even a closely related paper) without explicitly saying so would generally be considered self-plagiarism. Unless they have exceptionally permissive publishing agreements, you would be in violation of your agreements with both publishers if you didn't get explicit permission to do this. Even if the journals have the same publisher, they would probably still be unhappy, because this is a violation of scholarly norms.
If you feel your particular case is reasonable and would be considered acceptable in your field, then all you need to do is to be clear about it. Include a sentence like "For the convenience of the reader, Section 2.1 is copied verbatim from [citation to part 1]" and explain the situation to both publishers when you submit the papers. If they are OK with it (and you are honest with the reader), then everything's fine, but you certainly shouldn't submit the papers in this state without comment.
For this to work, you'll need permission from both publishers. In mathematics that would be an unusual request, and I doubt you would receive permission, but things may work differently in your field. In any case, you may come across as naive or eccentric if it doesn't work the way you expect, but at least nobody will be able to accuse you of being dishonest or manipulative.