First of all, I doubt this is a repost
Second, I would consider this as an exploit, but others may not. Inform me otherwise.
Okay, what you do is when you're copying your characters (any characters from any of your live realms, or pre-mades) is set up the copy for all the characters you want on the realm at one time, and quickly (because if you wait for them to copy over to the test realm, both horde and alliance, you won't be able to copy anymore, so I suggest getting all your copying done unless you want to delete your copies you just made).
Basically, what that means is get all the characters you want copied to the point of where it says "Character Copy Successful" and do it quick before any of the characters you selected to copy transfer over.
Example: 4 pre-mades:
Say I copy 1 horde pre made, I'll have to go back fast and copy an Alliance pre made ASAP, and if I want all my characters on the PVP realm I'll have to copy all 4 quickly so none transfer before I'm done getting my characters copied. So by the time I'm done I'll wait for how ever long it says for the characters to transfer (or shorter), log on to the Server and see Alliance/Horde Copied.
The reason I say do this quick is because if you wait to long and let's say 1 Alliance and 1 Horde copy and transfer over before you get the other 2 to the point of where it says it was successful, you won't be able to copy anymore on that server.
This worked for me both this current PTR phase (2.3.2) and last PTR (what ever that was that had pre-mades).
Let me know if I explained it bad because honestly, I had no idea how to explain this just right, so questions are always welcomed.
ALSO! This might actually work for live realm character transferring.