Surely there are cheaper ways, but you could use a HDSPe MADI FX or MADIface XT which have a very nice and
unique driver feature to mirror MADI BUS 1 OUT to the other 2 MADI buses.

You configure this in the RME driver settings. See handbooks of these two products and search for "mirror".

If you are not looking for a new recording interface then I think you could also use the MADI Router ("from memory",
didn't have the time to re-read the manual, browsed through that products manual a few month ago).
For my understanding these kind of things is one of the main use cases of the MADI router.
And this device also contains redundant power supplies for better resilience.

Only splitting would be easy - there exist optical, passive splitters, as well as passive (might not work) and active splitters (no example at hand) for coaxial. But what about the combining, at the card's input? That will require a complicated and expensive solution.

Hard to say without knowing what this device is. Usually, unused channels are passed through to the next device (MADI units are typically designed for daisy chaining).

Not sure what the problem was with my reply - it was fully correct. You could split the MADI signal and feed two full 64 channels to different devices. Of course your 'other' devices must be able to be configured to take specific channels out of the MADI stream.

Don't know if the Otari will pass on any MADI channels - check its manual to find out. On other devices either the whole stream is forwarded, or a configurable part of it. There is no generalization for this kind of feature.

BTW, the term 'split' is technically incorrect, I think. The 'splitters' I talked about 'double' the output signal, so one output is turned into two outputs, both having the same full channel-count signal.