Each fan is controlled discretely based on what is in need of cooling.
You can see this with a good temperature monitoring app like TG Pro.
If you suspect you are having a problem I would encourage you to install this app and post a screenshot of what it shows (sensors & fan) so we can try to figure out what's wrong.
Given the age of the system it maybe covered under warranty.
Download the App I pointed out to you it will clarify things.
The dual heat sink model has the heat sinks tied together by a common cooling bar to the two fan unit radiators which allows them to share some of the heat load from the discrete CPU & GPU.
Why are we picking this apart, you can't alter the design nor should you.
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