I hadn't tried it.. the offer T&C section on the thriwe site says -No, they are full plans with 4k streaming.
I stacked my Amazon prime membership which was about to expire in Feb 2023. Now it expires Feb 2024. Not sure why you're not able to stack it.
- In case you are an existing Amazon Prime member, you cannot redeem the code on a prime activated account.
- This code is transferable and users can use the code in another account where prime service is not activated