2 UART Connections to MCU using the Same RXD and TXD Lines
Use resistors to connect the GNSS module
When you want to program the device tie the GNSS end of the TX  resistor high



simulate this circuit &ndash; Schematic created using CircuitLab
connect the programmer at A and B and connec C to VDD so that the GNSS does not get confused by any responses sent by the MCU to the programmer.