Just finished testing with 4 Arduino Megas running in slave mode with an Arduino Nano as the master (tee hee... puny little Nano controlling those behemoths), each with 2 Hall effect sensors. Testing is going well so far.
In the first picture, the Nano is in the top left of the image, with a USB cable connected to it (going to the PC for serial I/O). The Megas are all lined up on the right side of the boards and the Hall effect sensors (8 of them) are along the left edge of the bottom left prototype board.
In the 3rd picture, you can make out the raw sensor readings on the tablet monitor window.