At last -- I think I've found the root of the problem.
After looking at the actual sample clock rate (vs. the specified sample clock rate), I found that there was a slight difference. I was specifying 345600 and only getting 344828. I'm not exactly sure why this was happening, but it is a big enough difference to matter when looking at sample times and such. After using the actual sample time, I was able to observe perfect synchronization between cards using the volt box switching to 5V. I call this the external test because the volt box is external to the system.
However, the "internal" test, where I command the analog output to 5V, still shows some (smaller) synchronization errors, i.e. the digital line switches to HIGH about 50 microseconds before the analog input shows any change from 0V. I'm looking into this and I think I'll be able to find the problem, but I just wanted to let you know.
I am noticing that the specified analog output rate is 2400 Hz but the actual rate on the card ends up being 2400.096 Hz. I have to think about it more, but this might be causing the "internal" test synchronization problems.
Thanks again for your help. I appreciate it!
Kevin