I don't know exactly what he means by having them both eat the apple. Simultaneously? Sequentially? The same day? One thing I do know: I wouldn't leave any pieces of that apple lying around where the poilishe shikseh might find it. By the way, the association of apples and love is not unique to our tradition: See, e.g., here.
Maybe this is connected to the passuk in Shir Hashirim 8:5
( Who is this that cometh up from the wilderness, leaning upon her beloved? Under the apple-tree (Tapu'ach) I awakened thee)
Despite the tone of this post, I want you to know that the individual that edited this book for Feldheim is a Talmid Chacham muvhak and an exemplary yarei shamayim. If there were more like him, we would be zocheh to Achishenu. With the current events surrounding the EJF, and the encouragement and occasional participation of allegedly honorable individuals in the mob-action lynchings of the principals of that organization (example: Rav Elya Ber Vachtfogel agreed to affiliate himself with the EJF after Rabbi Tropper resigned, and he was immediately called by the henchman of a certain gadol in Eretz Yisrael who explicitly threatened him with public humiliation if he didn't not only retract his acceptance but also claim to have never agreed to join the organization), it is only the adinus (tranquility) and the rei'ach Gan Eden of such people as this book's editor that clears the stink from my nostrils and protects my faith in the concept of of the spiritually salutary effects of a life of Torah.