Printing to different printers

Hi,

Does anyone know of any software (or how to write something) that would enable me to send different sized documents to different printers?

Both documents would be created at the time of saving, but one would be A4 and the other would be A5 or A6 (havent decided yet!).

I've tried using the same printer and setting the defaults for different trays, but no matter which way I set it up or print it I cant get it to work.

My apologies if this is in the wrong place - and if it is could a moderator, please move it to the correct one

Thanks in advance
Ian