2 things that should help. The different aspect ratio titles must be in different title sets, one for 4:3, another for 16:9. You cannot directly link from one title set to another, instead you must create a PGC in VMG with no programs, and a pre command to jump to the other title set. These are called "dummy PGCs". You then link back to the dummy PGC, which will jump you to the other title set. Here's a graphic that shows the allowed jumps/links/calls (jlc's)
http://mpucoder.kewlhair.com/DVD/vmi-jmp.html
Hope this helps clear things up.