More pins in Arduino

I would like to connect two Arduino boards, so I can control a ISD2500 addressing from an arduino board and . I need to have input for some sensors as well. Any sugestion? :


it is big and clunky, but the 74154 TTL address decoder will take four pins and decode them into 16 pins: you send it a 4-bit binary address, and the appropriate pins are set. I think there is a CMOS version-- the 4515?