Hi jon,
As PeterH says
If you're using stepper motors to control an absolute position, you will need some way to find/set the initial reference position. Maybe just parking the motor against a mechanical stop or limit switch?
The ATtiny85 only has 6 I/O pins so 4x to control stepper 2x communication leaves no spare pin to connect reference (unless someone has a bright idea). The ATtiny84 has 12 I/O pins so may be more suited.
Each ATtiny85 is programmed with its own unique 6-bit address number (0-63), and listens for 2-byte instructions over a common serial interface from the central Arduino. The first byte of info is the address, the following byte is a command to turn x number of revolutions in y direction, which the selected ATtiny85 translates into instructions for the ULN2003 board using the "stepper_oneRevolution" code.
You should also include a command to make system calibrate as per PeterH quote.