Current test is done on breadboard. One for the arduinos is reading a 24VDC sensor and counts postcards in stacks of 24. once 24 is reached it turns on pin 13 and restarts the counter. The second arduino receives high input from first arduino and triggers one realay to close 24VDC circuit speeding up a conveyor belt for 1.5 sec creating a gap in a stream of products. At the same time opening another relay to interupt a signal from the machine that normally would trigger the motor drive to step the conveyor belt forward.
Based on this test I laid out a circuit board using Fritzing. it has footprint for two duinos, six relays, power plug for incomming 24VDC, selector for 8 ground connected resistors used for inputs as well as bunch of holes for point to point soldering.