Automaticly switch and fallback RS232 async/sync ports

Hi,
I have two pc hardware (main and backup) connected a mechanical switch box which switched these pc's a modem.

Swich box has 4 input(2 db9, 2 db25) , 2 output (1 db9, 1 DB25).

My modem communicating with pc (main or backup), by switch box (selectable main or backup) async or sync.ly

Now, I want to make this operation automaticly.

My switchbox should be change its state automaticly and route backup ports to modem successfully. And fallback main ports in normal state.

Is there any design, schematic, electronic components (crosspoint switch, solid state relay OR analog switches (TS series) with model etc.) or any advice for this electronic switch box.