each sim card is attached to the phone it came in. If the extra sim is inactive then XM can activate it with the phone you want and inactivate the other sim. It is more easily done if the phones came from XM. I was lucky to find a customer service person that was able but more importantly WILLING to take the time involved. The sim has long number on it that they can match up with the IMEI and match it all up on your account so your phone works and the right phone is shown on your account.
I had an inactive X Charge left from an upgrade to a iphone 7, my wife hated the iphone and wanted her X Charge back. XM no longer sold the X Charge and the first cs person said nope, nada, no way. New phone, new line, new number. I called again explained my problem and she said she would call me back the next day, and she did. She worked her magic and my wifes LG X Charge now shows on my account as a LG K30 BYOD, same phone number and everything works. Do I care if it's not showing as an X Charge, nope. Since the beginning XM has been very good to me.