First Considerations:

1, Download and install the latest driver for the StudioLive and OS here (with both boards disconnected from the computer and each other!): 

http://www.presonus.com/technical-support/downloads/drivers-software/

2. Connect each board INDIVIDUALLY (using the boot up method below) Open Universal Control and follow the update instructions of the software.

3. Make sure you are using an excellent quality Power Conditioner (Furman makes very good ones) Without stable power, sync can be problematic and/or your sound quality can suffer due to noise.

Now, (with power to all components off) hook up the 16.4.2 with the lowest serial number to your computer via your 6 pin firewire cable. (this is the Master)

Next, hook up the 16.4.2 with the higher serial number via firewire to your Master board.

Then, power on your computer.  Make sure to let it boot completely up. Log in and let your system load its Startup programs for a moment. (give it 1 minute after login, at least, just to be safe)

Now reach behind both mixers at the same time and turn on the power to both simultaneously (or as close as you can get)

Your 32 channel StudioLive system should be up and running now!

Note: Shutdown is the exact Reverse of the above procedure!!!

have fun.