Hold on. Somethings not right here. Marvel didn't solicit any such book. SuperHero Enterprise(Hero's World) solicited a book to be signed by Stan and Carmine Infantino but Carmine got fired before the book came out and he chose not to sign any, at that time.
It is possible that Julius Schwartz signed it at the show, but the story is wrong about it coming from Marvel with the two signatures.
The book was delivered to people who paid only signed by Stan. some got a letter explaining what happened, others just got the book.
Edit-- It seems Carmine did sign some books, up to #431, which makes this book even more suspicious as it's numbered 310. Carmines signature should be right where Julius's is.
From GCD:
According to Overstreet, there are 5,000 numbered issues signed by Carmine Infantino and Stan Lee. Based on information gleaned from several internet postings and sales, it appears that only 2000 copies were in the signed edition. Furthermore, because Carmine Infantino was no longer editor at DC as of the end of January 1976, it appears that only a fraction of the 2000 copies were signed by him (at least from #431 up do not bear Infantino's signature). All copies were signed by Stan Lee. Overstreet's claim of the signed copies being a second printing is not supported by any data on the comics themselves.
Between the phony backstory and JSA's near worthless authentication, I wouldn't pay $50 for this book. Sorry.