Transferring characters between B.nets

Hey all,

I have a question regarding transferring characters between different accounts.

A guildie swapped characters between him and his brother a few times, same last name obviously. They do have achievements and FOS shared, but the mounts didn't actually become useable.

my question is, is this because the character that got migrated didn't actually loot the item, or will this always happen when a character is migrated between different BNET accounts?

Like, if i were to migrate all my characters to that BNET, would i be able to use the mounts or not?:D

thanks in advance