iMessage is a completely independent service from either iTunes or iCloud. You cannot have two iPhones or two phone numbers assigned to the same iMessage account.
If you try to use the same AppleID for iMessage on two different iphones then only the last one to sign in will have its phone number associated with that iMessage account.
The only way to sync imessages across two iPhones would be to use a common email address as a receiving address. But if the sender(s) uses the phone number to send to, then the messages won't sync.