Bad news -- this seems to be an inherent restriction for that particular digital module.
I tried replicating your system with simulated devices, saw the same error, and tried a number of additional DAQmx signal routing "tricks" to try to prevent it. None of them worked.
So I did some more thorough searching on the site here and found this. It looks like you'd need an additional digital module. Or perhaps a different one -- it sounds like the restriction is for "serial bidirectional modules". I'd venture that the high channel count is probably why the 9375 is serial (and thus restricted) rather than parallel (and perhaps *not* restricted?).