The Saffire Line is capable of Dual Unit Mode in which two Saffire are connected via Firewire.
In this mode the two Interfaces become one interface. You will not need to create any aggregate devices. You will only need to select one "Saffire" in your DAW's preferences.
Many users use this mode to get 16 analog inputs via two Pro 40s.
In this mode the inputs of both units are combined together, offering you 40 total inputs. (This would be different if using interfaces other than two Pro 40s.)
Since each unit has a total of 20 inputs and 20 outputs you will not set up your Analog Inputs to your DAW as 1-16.
The first Pro 40 will have it's analog Inputs as 1-8. Inputs 9/10 will be that unit's SPDIF, 11-18 will be that unit's ADAT inputs and 19/20 will be that unit's Loopback Inputs. These will always remain, even if you have nothing plugged into those digital inputs.
The second Pro 40's analog inputs will begin at Input 21. Analog ins will be 21-28 followed by that unit's SPDIF on 29/30, that unit's ADAT on 31-38 and that unit's Loopback on 39/40.
Please make sure if you are using two Pro 40s in Dual Unit Mode you set up your DAW to accept from Inputs 1-8 and 21-28, not 1-16.
Further details on our Dual Unit Mode, including Input Tables for all other configurations of Saffire Interfaces will be in our Dual Unit Mode User Guide.
***Please be aware that which unit is considered "first" or "second" is not determined by the order the cables are plugged in. The unit with the lower Hardware ID will always be first. The easiest way to determine the Hardware ID is to plug each unit in alone and go to Saffire MixControl > About Saffire MixControl. Figure out which unit has the lower Hardware ID and label them for yourself to avoid confusion about which inputs are 1-8 or 21-28.***