No... the menus are the exact same... disc one has chapters 1-28 and disc 2 has chapters 29-61, all the chapters are in the chapter select menu but if you try to go to a chapter that isn't present on the disc it will either play from the last chapter on or from the first chapter on, depending on which disc is playing. The ifo file of disc 1 has chapters 1-28 in the pgc chain and disc 2 has chapters 1, 29-61 in the pgc chain. after merging the vobs it was a major pain in the butt to add chapters 29-61 to the ifo of disc 1
now that was way off topic but I believe a similar approach could be taken to fit 2 different movies onto a single disc.
all the new menu would need is two buttons, one for the track 1 menu and one for the track 2 menu. I am sure that both menus could be stored in the VTS_01_0.vob and reference each menu with a unique ID
currently there is no software out there that I am aware of that can do this but if a codehead guru can come up with a program to edit menus (like menuedit for example) then surely one can create some sort of software to accomplish this task
I say again, anything is possible