[DiapDealer]

Yeah, you're going to want to get over that "But Dreamweaver can..." habit you have. As BR said, Sigil is not; nor was ever intended to be; nor is ever going to be a Dreamweaver clone/replacement.

To answer your question: No. Book View and Code View will never be viewable simultaneously. Since they both have the ability of modifying the other's contents, having them both open at the same time would be like a snake trying to eat its own tail.

Book View is on life support. The only significant changes to it anyone can expect is it going away some day.
(though there's no official plan or timetable for such a thing happening -- before anyone unnecessarily panics)