If you need a second SIM then you need to use a different email address to the one associated with your current 48 account (if any), register as a new user, get a new SIM and follow the usual activation/getting started steps:
In case it matters to you there is no facility on 48 to manage more than one SIM using a single account - every SIM requires its own account/login/email/number/membership.
"Unlimited" data on 48 means 200GB per month with a fair usage policy.
If you use up the 200GB allowance before the membership month is up then you'll need to buy a data add-on or use Flexi Data to get more data for the rest of the membership month:
I thought that Amazon Fire TV Sticks used wifi only and not mobile data/SIM?
What make and model of stick are you planning to use?
