You don't get another left hand, or lung... No you don't get another twin flame. A TF is quite literally the other half of your soul, to put it bluntly. If you feel you've out-grown this person I doubt that they are your TF. A soulmate, absolutely! And they are here to teach us lessons and sometimes move on, other times they stick around.
I can't see anybody "growing out of" a TF relationship. Especially when the purpose of such a joining is usually learning and acceptance. Think of unconditional love and understanding (sometimes after the argument) between two people. A bond that no matter how far or how long separates, is still fresh every time they see each other.
Out-growing soul-mates happens. Especially once the lesson or value they came to teach has been acknowledged. Don't fret, though! Many more enchanting people will find their way into your world