Can you legally get a clone of your SIM card?

So I have two T-Mobile phones. The first one is a Note 3 and it broke about four months ago so I bought a Galaxy Avant. I was able to fix my Note 3 and I use it mostly for watching movies, playing games, and taking good pictures. I use the Avant because it has a smaller screen size and its a good phone to use for just texting and calling but sometimes I would prefer the note 3. Its annoying to switch the SIM card back and forth between the two and they both don't have the same SMS messages. My carrier is Family Mobile and both phones are compatible with Family Mobile because they use the T-Mobile network. I want to know if I could have two SIM cards for the same line so I don't have to switch the one card back and forth a lot. I know switching the SIM card back and forth is allowed and not illegal but if I had a clone would that be legal? And would Family Mobile clone my card for me?
Added (1). And if anyone knows any way to sync SMS messages between the two phones with or without an app then that will be great. I'm looking for a way to sync apps automatically.

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