|You can use an intelligent T Switch.|
These detect print arriving and switch and hold for a predetermined time on the incoming port concerned.
They are available for:-
RS232 (9 or 25 pin) - Parallel - usb ports.
With (the larger the better) or without buffer
2 or 4 ports, maybe more
Googling 'rs232 t switch' produces hits such as:-
As an example see Switch 85643.
As a matter of interest, where are you based? I am in Hammersmith, West London.
Good Luck - Keep us posted.