Thanks for the reply, I have enabled a second one and it's now set in configuration.h though only the TFT is working OctoPrint doesn't want to connect.
You still have to use the correct values for serial ports.
As indicated by the comment right above the setting, one of them needs to be -1 in order to have a serial connection on the USB port:
/**
* Select the serial port on the board to use for communication with the host.
* This allows the connection of wireless adapters (for instance) to non-default port pins.
* Serial port -1 is the USB emulated serial port, if available.
* Note: The first serial port (-1 or 0) will always be used by the Arduino bootloader.
*
* :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
*/
So, you should set one to -1 and the other to 2.
It's also worth noting that the other serial port number greater than 0 correspond to physical serial ports on the mcu. For BTT boards, you can usually determine where those are wired to by looking at the pin diagrams. Any RXn and TXn pins are serial port n. For example, the TFT is indeed serial port 2 in your case:
