Please Note: Before attempting to use mutiple Saffire PROs, you must ensure that both units have the same up to date firmware, and are running at the same sample rate. You may also want to disable ADATs to save CPU, as running 2 units will use more CPU that running 1. In this example, the ADATs have been disabled (providing 2 x 8 analogue ins and outs + 2 x SPDIF ins and outs)
In order to use multiple Saffire PROs within Mac OS X, it is necessary to create what's known as an "Aggregate Device" within Apple's "Audio MIDI Setup". The "Aggregate Device Editor" is available within the Audio MIDI Setup utility. This allows you to combine multiple sound cards into one "aggregate" sound card, where the inputs and outputs of each device are included in the aggregate device.
Example setups for Ableton Live and Logic PRO are shown below. The setup in the DAW may vary slightly depending on the DAW you are using. Normally, these configuration options can be accessed from the preferences menu where you select your DAW audio driver and device. You simply need to select the "Aggregate device" as your new sound card.
You will now have access to all your inputs and outputs for both Saffire PRO units in your DAW software.