RAF doesn't make a difference, whether its on one B.Net or not.

For RAF purposes, all that matters is which individual account invited the new RAF account.
They can be on the same or on different B.Net accounts.

One thing to note, if you add an account to a B.Net email, then you cannot do character transfers to or from that account, for 30 days if the destination/source is a different B.Net email.
But if the destination/source are both on the same B.Net, then you can do transfers immediately.

I'd recommend one different B.Net email per Warcraft account.
And a single authenticator, for each warcraft account.

You can RAF from Account 1 to a new account.
Whether they're on different or the same B.Net; that makes no difference.