You are not allowed to use multiple free trials and that is true for any of the free trial promotions that Apple provides. You can see that in the terms for the offer here. They reference qualified returning subscriber in cases where a person has used a previous trial before that was for a shorter period of time. For example if you used a 1 month free trial before and this offer is for 3 months, you would only get the remainder of 2 months free that had not been used. You cannot stack the free trials. Check out what the "1" refers to in the subscript of the offer:
New and qualified returning subscribers only. Plan automatically renews at your region’s price per month until canceled. Only one offer per Apple ID and only one offer per family if you’re part of a Family Sharing group, regardless of the number of devices you or your family purchases.
https://offers.appletvapp.apple/tvp01213mf
You can also see the terms for all offers Apple provides here:
Offers - Apple
You can view the terms of all promotions in your region as well. Choose your region on the top of the Offer page.