Connecting 2 external PCBs with a 1m cable
I tested it, and it works (115200 baud rate), there's just a small
  ~20ms latency getting a reply when sending a request from PCB1 to
  PCB2.


That latency is almost certainly nothing to do with the data rate of 115 kbaud. In 1 ms there can be 115 bits transmitted so, unless your message protocol requires thousands of bits, you are going to be stuck with that latency.


  So I was told I could use 2 additional wires and communicate using SPI


and


  On each PCB there is a MCU and each MCU is able to send and receive
  commands from the other one.


Not all that easily achievable if you are wishing to swap who is master and who is slave. I'd stick with UARTs and dump the RS232 bit - just use logic levels. SPI and UARTs will generate EMC and there's no getting away from the fact that for equal baud rates, the EMI will be very similar.