Multiclassed as two spellbook-based casting classes - can I share a spellbook?

[Genuine]

Say you've got a wizard/magus. Pretty much all of his magus spells are also wizard spells, and each side can also learn spells from the other's book. Can he cast out of the others book?

Would a wizard/magus need two spellbooks, or one? And could he scribe Invisibility into his book and prepare it to either class? (Assuming both could cast a second level spell, but you get my point).

[James Risner]

By RAW all the spells from Class A could be consider foreign spellbook spells by Class B. You would need to "learn" then but once learned you would be able to prepare them.

Now, the other point is the spell needs to be identical on both classes. So if you put a spell called happy times in the book as a level 2 Wizard spell, but it is a level 3 Magus spell you would need to copy the level 2 version as a Wizard to the level 3 Magus version.