This applies to the Current Saffire Range
Computer with a single FireWire port
Connect one Saffire to the FireWire port on the computer. Connect the second Saffire to the spare FireWire port on the first Saffire.
Note that the Saffire PRO 24, Saffire PRO 24 DSP and Saffire PRO 14 have a single FireWire port so, both need to be connected to a Saffire PRO 40 or Liquid Saffire 56 and will require the PSU when connected in this way.
Computer with multiple FireWire ports
Connect both Saffires directly to the computer.
Note that both FireWire ports on the computer MUST be on the same FireWire bus. i.e. the same PCI / PCIe FireWire card.