Which input pins can I use on the ESP8266 NodeMCU
GPIO2 is NodeMCU D4 which is U1TXD, transmit for the second UART, generally available. GPIO3 is NodeMCU U0RXD, receive for the first UART, generally available if you aren't using serial. GPIO1 is NodeMCU U0TXD, transmit for the first UART, generally available if you aren't using serial. GPIO9 is NodeMCU SPIHD and is NOT available, as it is required for the flash memory. All that is well documented in many places. You can easily use serial to move data between modules, or multiplex your input to the one module. Cannot provide much insight without more details.