Normally SAB duplicates any
\mt1/2 titles that appear before the book introduction at the beginning of chapter 1 as well, and that’s great and appropriate. But there are cases where we might want the title(s) to be different. For example, a shared introduction for 1TH and 2TH appears at the beginning of 1TH, so you want that title to be “Letters to the Thessalonians”, but you want the title at the beginning of chapter 1 of 1TH to be “First Letter to the Thessalonians”. (See more detail in this post: possibility-of-different-main-titles-mt1-2-for-intro-and-chapter-1.)
It appears that the current functionality is that SAB collects and remembers any
\mt1/2 markers that are found in the introduction, and then spits them out again at the beginning of chapter 1.
My proposal is that if SAB sees any
\mte1/2 markers as it is processing the introduction, it throws out the titles that it collected before and starts collecting
\mte1/2 markers. (You would need some sort of flag to indicate that it is now collecting
\mte1/2 markers, as you wouldn’t want a second marker to throw out the first line collected.) Note that the
\mte1/2 markers would not be output where they are located, but just saved until chapter 1.
\mte1/2 markers would be allowed either before or after the
\c 1 marker, but I think normal SFM would put them before the chapter marker, so if you only implemented that case, I think that would be acceptable.