Two Arduinos, RFID readers and Unity

Dear All…

I’m trying to connect two Arduinos to the same computer, and both have six RFID readers connected to them. I’m trying to build a videogame using Unity that uses the data sent through the serial port coming from the RFID and I managed to do it with one Arduino Uno.
The problem is I need to use two boards at the same time and I don’t know if it’s best to buy another board or extend the pins on the one board i have with many MCP23S17 expanders.

Thank you.
Best regards, Nicolò