Well I can definitely say the SPDIF ports aren't for daisy-chaining, since the standard only allows 2 channels of audio to pass!
Not to sound like a jerk, but I ran a pair of Firepods like that for over a year. That is the only way daisy chain them for an additional 8 inputs is to use the S/PDIF connection. In most cases however, you are correct, the S/PDIF is only available for 2 channels. See below.
http://presonus.com/media/manuals/multiFPhowtoXP.pdf
How to use up to three FIREPODs with Windows XP
You can use up to three Firepods on the same system over a single Firewire connection. In order to use multiple Firepods
together you must first download and install the latest driver, and run the firmware updater included with that driver for
each Firepod. Once the drivers and firmware are up to date, using multiple Firepods is easy.
Control Panel:
The control panel includes drop down menus to select
samplerate, latency, and clock source, as well as an Active
Units Panel.
Which Unit is Which?:
To determine which position each Firepod is in, you will
need to switch the clock source to SPDIF 1, SPDIF 2, or
SPDIF 3 and look at the Firepods to see which one has a
sync light that is flashing red/blue, indicating that it is
searching for SPDIF sync (make sure no SPDIF
connections are made yet). The number behind each
SPDIF selection refers to the position of the unit in the
Active Units Panel. The position of each unit will
determine which set of inputs/outputs the unit provides in
your DAW software (ex; position #1 = inputs/outputs 1-
10).
See bro, I know what I'm talking about some of the time