If you are modeling the F-15 with the main gear doors closed, you can glue in the shorter (outboard) door before the landing gear, as shown in the first photo.
Of course, first you have to remove forward section of the door, first.
The second photo is to show that you need to make a small notch on the longer (inboard) door. If you glue that door on, you can no longer install the landing gear. Or at least you will need to figure out how to work it in (you perhaps could notch out the hole that the gear goes into, but I have not verified that myself and it will make it harder to properly align the gear).
The way the main gear doors work is common with the P-61B/C/D/E.