Karyn
You will never get two independent clocks to stay in sync with each other. The manufactuering tollerances just won't allow it.
The only way to use two or more interfaces together is to have them both sync to a common (single) clock source. That normally means word clock.
The same also applies to any digital equipment you connect to your system digitally. So if you have outboard hardware processors that have digital interfaces, ADAT, AES s/pdif then they must also be fed the common clock signal.
Depends on the precision required. I once recorded a continous 1 hour video shot using an old Sony Hi8 video recorder and recorded the sound digitally on a separate setup.
On mixdown I had perfect lip sync from beginning to end.
Multitrack needs totally perfect timing. I'm worried that in a 3-minute pop song, flanging may be heard.
Naturally a word clock/ADAT is the ideal, but my interfaces do not have that. Perhaps the s/pdif may work. The Tascam 1641 allows the user to select the external digital in as timing master, and I have an optical-to-electrical converter, so I can get the Edirol to work with the Tascam.
Its going to be an interesting weekend. I'm sure I'm not the only person needing more channels, so I'll post my results.
Edit: Thanks for the added info, FBB. Will try it the unconnected way, too.