If you're using OSX 10.4 (Tiger) or above, the answer is yes. In fact you should be able to combine any two devices that support CoreAudio. This version of the Mac OS makes it possible to combine the inputs and outputs of two or more soundcards into what is called and 'aggregate device'. This can be done in the OSX AudioMIDISetup utility. Instructions on setting up an aggregate device can be found in the Audio MIDI Setup Help file, and also here.
It is important to note that SaffireControl does not support control of two Saffires simultaneously. SaffireControl believes that there is only one Saffire in the universe. However, if you want to use them simply as sound cards with ins and outs, you can link them together using Tiger's aggregate device editor.