I think this is a version of Mikes advice, use an external interrupt for detecting which Slave is to send the data, have the same bus for data transmission.
Alternately if you are using a MAX485 IC then connect the bit to each of the DE and RE inputs turning the required bit on for each specific IC( you will require an inverter here if you can not spare 2 bits for each IC.Or you can even use a decoder for that purpose, use 3 bits and connect them to the Decoder, using a not gate do it.