What is the least complex way to implement a DP3T ON-X-ON switch?
If you can guarantee that all the TX outputs will be tristated (floating) when the switch is thrown, then you can use a 4P3T slide switch wired as follows, for the given switch:


1 and 3 to MCU 1's TX
4 and 7 to MCU 2's TX
5 and 8 to the FTDI's TX
2 to 10
6 to 14
9 and 12 to the FTDI's RX
11 and 16 to MCU 2's RX
and 13 and 15 to MCU 1's RX


P.S. the lack of a schematic is due to CircuitLab not having appropriate symbols for a 4P3T switch.