At Step 2 of the registration process, you'll simply have to mention which other collecting society your co-authors/publishers are registered with. Obviously, they will not receive the List of Works, which is a Soundreef document.
If your co-authors/publishers are registered with SIAE, they’ll register with form 112, in which you will be quoted as not enrolled in SIAE. However, you don’t need to provide any documentation to company.
It’s worth mentioning that the division of ownership of the shares is an agreement between private parties - you and your co-authors/publishers – that’s made before the tracks are registered with a collecting society.