Per the Change Request Guidelines, "Since a Flipbook is a single entity, we can only allow a single place holder (or folder) for a flip book issue. The cover with the Indicia should be the title that the flip book is entered under. If there is no indicia or there are indicias for both sides, then please enter the book under the title that contains the most pages in the story as this is denoted to be the main story."
As such, this issue is invalid and should be cataloged under Ultimate Spider-Man (Vol 1) 87-B.