First thing you need to realize is that supplying an external clock source to a converter does NOT eliminate that converter's internal clock from the picture and does not -- in normal circumstances with modern gear -- "improve" internal timing but actually tends to increase jitter as that converter's internal clock must constantly attempt to "correct" itself to stay in synch with the external clock source.
Obviously, if you want to use more than one converter simultaneously, you have to synch them. In a complex, multi-converter rig, it
may be more convenient to use an external clock source (like a Big Ben or other) with a star topology but in general, a properly terminated daisy chain using the internal clock of the first converter in the chain will tend to cause the least jitter in the synched converters.
Unfortunately, there are a number of people who -- for whatever reasons, some of them are celebrity endorsers and others are simply people who bought the Kool-Aid and apparently figured they then had to drink it -- claim that a "superior" external clock will "improve" the accuracy of a converter's sampling.
The science does not support this.
You can read all about it in what is undoubtedly the epic thread on the topic -- and you can read some amusing rejoinders from reps of a certain external clock company, too:
PSW Recording Forums: Dan Lavry => Proper word clock implementation
NOW... if someone says they LIKE the sound better with external clocking -- no argument possible there... it's only if they claim it improves something measurable like accuracy, that they're crossing the line.
And both Dan Lavry (in the thread above) and Digidesign (in their whitepaper on clocking issues) have both suggested that those who LIKE the sound of external clocking are entitled to their subjective judgement but that an implicit conclusion from such a claim is that they may simply LIKE the sound of increased jitter.
Anyhow... this ish is, like, my
bête noire...
And, amusingly, I used to run a rig with an external clock going to two converters, but it was an old ADAT system that was designed that way [BRC supplying dedicated clock via BNC].
I got caught up in it when a certain external clock maker and their celebrity endorsers started promoting this "improve your converter with external clocking" rigamarole. That didn't sound right to me (from my limited understanding of the issues at the time) and I went looking for answers. Eventually, that led me to Dan Lavry and the Digidesign whitepapter and... well... now it's my pet peeve for reasons that probably have much to do with personal pathology, I'm sure. (FWIW, I use a MOTU 828mkII, which went from everyone's darlin' a few years ago to can't-get-no-respect status -- and that seems to provoke SOME folks to want to mod it or try to improve it in various ways. Me... I figure it is EXACTLY the same unit it was when everybody thought it was so groovy -- so THEY must have changed.
And I looked into the modding issue, too, and did not much like what I found. If I was that unhappy with it -- I'd abandon it rather than trying to magically "fix" it somehow.)