Use the chain that works the best!
As regards the two computers, if you don’t have a digital link between comp 1 and comp two then there’s no advantage to linking wordclock between the two. If you do then it should be viewed as whatever card you determine to be the “Master” card then all clocks should be derived from that.
There are word clock generators which may help with jitter.
The limitation is most probably a driver addressing problem (not to mention you would need more that 4 PCI slots), so you could theoretically have two computers with four card each if you wish. Spdif is also capable of providing clock for syncing other cards, I presume you can select the clock source in the cards driver panel, although wordclock would be the preferred clock. As pointed out above a master clock would be preferred for multiple wordclock setup rather than chaining, but hey if it works then thats all that matters. 
As I understand both comments, a ‘Word Clock Generator’ & ‘Master Clock’ are the same thing
Not exactly…
A Word Clock Generator is a discrete “box” that generates timing signals, often in addition to word clock. Other devices “sync” to its timing signals.
A Master Clock is not a specific device, it is a function provided by some device that the other devices sync to. The device that serves as the “master clock” can be a discrete word clock generator, an audio interface, a component of a video chain, or some other source.
In order for a digital system to work properly, all of the devices in the chain must use the same clock source to avoid, to use a broad example, one device starting to count at the exact second and another starting a half second later.
Many digital devices have built-in word clocks, and can serve as a master clock, the quality of these clocks can vary. For this reason (among others), a separate word clock generator is often used in more sophisticated facilities. While most discrete word clock generators have multiple outputs for a given format, generally a single output is used to connect all of the devices that are feeding a single computer. The other outputs are typically used to feed other digital channels, such as might exist in a multi-room recording studio or post-production facilities.
Hope this helps clarify things a bit…