Best way to connect to UART lines to multiple entities
The MCU Tx can be connected to both Rx pins without any problem.

For the MCU Rx pin there is a problem, because you can't connect two outputs (FTDI and SENSOR Tx) together. Some solutions


use a two position (three pin) jumper
put a resistor in the line that is to be overrruled by the other, presumably the sensors Tx line. unplug the downloader when you wnat to use the sensor.
put a gate (AND port) between the two Tx lines and the Rx. Make sure than an unconnected input (when the downloader is removable) is pulled up to a 1 level.