thanks for helping me a little.
To answer some of your questions: Since I do not have an external trigger cable for the DMM, my trigger source for the DMM is, like you expected, a RTSI data line on the backplane that picks up a generated digital signal of the HSDIO that I connect as external trigger to the PXI-5404 it's PFI input and that is send on on the RTSI data line (pretty complicated but I see no other option without that external trigger cable). It's not attached in my code since I use a separate VI for that.
The whole idea of my program is to only send a finite amount of digital data (not continuous) to the DMM so that the DMM only captures the data 1 time (at a trigger edge - rising or falling - of one of the generated digital signals).
And, like you point out, there's probably a timing dependency issue between the HSDIO and DMM, but how can I solve that issue? Putting them in series probably won't help me either? How can I make sure both devices are INITIATED at the same time? I can't seem to find a good way. It sometimes works at very low frequencies (1 or 2 KHz) but it should also work at 50KHz or 100 KHz I guess (since the bandwidth of the DMM is 300KHz.)