You can have a music subscription which is linked to your account activated on only one phone at a time.
If you have a voucher from your Nokia 200, you can use that voucher code to renew the subscription on your Lumia 720.
If you didn't get a voucher code, and you just have to sign in on your Nokia 200 to activate the subscription, then you can't use it on your Lumia 720.
If you want, you sign out of mix radio on your 720, and then sign in on you Nokia 200 using the same username as you did on the Lumia.