I would prefer an optical MADI connection to a coaxial one to be sure of galvanic isolation.
https://www.rme-audio.de/rme-madi-technology.html
https://forum.rme-audio.de/viewtopic.ph … 898#p82898 (note: MC refers to MADI coax here)

You need an appropriate multimode transceiver on both 12Mic-D's, preferably two from RME (for each of the 12Mic one), so that you don't accidentally get one with the wrong wavelength or non-functioning firmware (for other devices).

You can use either OM3 or OM4 multimode fiber cable. I would choose OM4 and establish this as your standard (to not end up with a mix of OM3 and OM4 cables, stick to one type in one installation). Both work well, OM4 is the newer standard, allowing for longer cable runs for 10G networking. It's about the same in terms of cost, so take this. Purely subjective: the standard violet color looks nice.

Due to different connector standards (on the UFX III SC, on the SFP/transceiver LC) you need different cables. A duplex cable SC-LC and a single or duplex cable LC-LC between the two 12Mic's.

With optical fiber cables, please pay attention to the bending radius and do not kink the cable. Otherwise, the cables are easy to handle, and I also use them. There are also 0.5m patch cables that work well in a portable rack with 3-4 height units. I recommend 4 rack units for 3 devices so that heat does not build up between the devices and can dissipate better through normal air circulation.

Here an example: UFX III, 12Mic, Octamic XTC, the only difference here, two devices have SC plugs (UFX III and XTC):

https://www.dropbox.com/scl/fi/wc4kibjd … k&dl=1

Please note: you can bend the cables a bit more so that it fits into the rack, I left some room as I do not see anything of the cables anyway, as it is located on the back. Regarding the rack: you only require 23 cm depth, then it's easier to reach the plugs at the devices.