Depending on your OS I think there must be sync for them to be able to work together or only the first device will operate.
Have you had a look at the Aardvark support group site?
That seems to give the answer doesn’t it ?
I was going to ask if Steve’s cards support multiple units like my Motu one, and it does as long as he has 2 PCI cards as well as the breakout boxes.
How many instances of the Control Panel for the cards are there?
I have been looking at Echo Layla 3G and from what I can tell, you can have up to 3 devices without needing any form of external sync.
Word clock will produce more reliable sync with less jitter I believe, with the result being that there is less pops and clicks in audio material. I don’t think phase or frequency response is effected but I’m unsure.
It could free up some I/O if you use Word Clock exclusively.
The spdif chaining may be unreliable when chaining 3 or more units together, word clock would be the first choice for good clocking.