You can use two Ensembles together in two ways.
1 - One unit connected to the Mac via Firewire and the second unit in Standalone Mode over Optical connections. This is the most stable method to avoid clock issues, as you can clock the units properly.
2 - You can also connect both to the Mac via Firewire and use the Apple Aggregate Device Mode. This may be less stable, even if you clock the interfaces together over Word Clock. Aggregate Device Mode can be more prone to clicks and pops.