If you're talking about receiving messages on two separate iPhone devices, this can only be accomplished if both devices are sharing the same Apple ID for iMessages. If so, messages from other iOS devices will be received on both devices. You could also enable text forwarding. This would also require the same Apple ID signed into both iMessage accounts on the devices. I'm not 100% positive if it works between two iPhones. I have it set up on my iPad and my iPhone. I would assume it does but I can't confirm that.
Hopefully someone in the forums has attempted this between two iPhones and can provide more information.