Yes, this is possible, but there are several things to keep in mind.
First you DO need a mic splitter. The Zoom F8 does not have direct outs for each channel, so you would need to split the signal for each mic. So that’s potentially an 8-channel mic splitter you would need. Radial makes a great one but it’s very expensive. ART makes a usable one that is more affordable. EWI makes a very good 4-channel unit that you could get 2 of. You would run a cable from each mic to a channel of the mic splitter, then use the “Direct Out” or “Pass Thru” from the splitter to the Zoom F8. Then use an “Isolated Out” for that channel to the Tascam DR680. You should only need to send phantom power from the Zoom. Levels would need to be set on both the Zoom and DR680 separately.
Also on the DR680, tracks 7 & 8 need to be fed digital audio to be able to record separate tracks rather than just a mixdown of the other 6 tracks. So you need something to convert your analog mic signals to SPDIF or AES/EBU audio to feed channels 7 & 8. The DR680 is also a little bit picky in which devices it will accept SPDIF from, due to copy protection schemes implemented in the unit. I have successfully used a Focusrite 6i6 to act as a preamp and then send SPDIF to the DR680, so the 2-channel version should also work. I also have an Apogee Mini Me unit that I have used to do the same.
Bottom line: it’s possible, but not exactly easy.
Have fun!
Rob Neidig
R&R Media Productions
Eugene, Oregon