1) Can I put "Ibid." at the TOP of the footnote section when referring to a previous book, which full title is placed immediately before this "Ibid." but on a previous page?
If you decide to use a continuous footnote method (from the beginning to the end of your document without any per section division), then you can use the "Ibid." even if the full title of the source appears on a previous page. But, if you use a per section footnote method (the first footnote of each section starts over at number 1), then you can use the "Ibid." when the full title of the source immediately appears on a previous page (must also be the previous footnote) but not if it appeared in a previous section's page; in such a case, you would have to include all of the required footnote information.
2) Do you use "Ibid." to refer to a previous book ONLY or to the WHOLE previous footnote and what if this particular footnote regards two different authors (2 books)?
"Ibid." is to be included whenever an identical reference to the previous footnote is used; the page number can be the same or can be different (in which case, you would indicate the different page number). Now, if the previous footnote regards two different authors (2 books) and the next footnote regards only one of the two previously cited authors and/or books, it is not an identical reference and "Ibid." should not be used. Hence, you should include a fully referenced footnote instead of using "Ibid.".