It is probably not necessary to match the 75 Ohm cable if you can make its
electrical length a multiple of a half wave on each band. The additional loss
due to the SWR will be negligible, though there may be a slight reduction
in bandwidth. The impedance at the transmitter end of the 75 Ohm cable will
be about the same as seen looking into the 50 Ohm cable at the switch.
If any of your antennas can be matched to 75 Ohms, you could
use 75 Ohm flexible cable to the remote switch. That would make the SWR
on the whole cable low. Tuned power amplifiers should have no trouble matching
75 Ohms. Solid state rigs without antenna tuners might be a bit unhappy
if the SWR on the 75 Ohm line gets above 1.5:1 or so.