Why do you prefer to use two different Arduino? If there is no specific reason for that you can use one instead of two. Just check the time of each transaction. Let's say user punched it's card several time within the day, you can follow these fingerprints for determining how much time he/she spend in the desired field.
I thought as you when I started the project but when I spoke with the customer he said to me that the users prefered two different place to check-in/out. So, the Arduino send a request to the backend in which its operation type is included.
When I finish the project I can to do your propose☺️ because I prefer too.
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