Replying to myself in case this is helpful for others. Here is what I have learned...
Tenet Magic Box - used to allow the easy connection of an aftermarket speedo to the factory wiring harness. Unlike the Motogadget M Unit, this does NOT replace the guts of the bike (relays, fuses, etc under the seat). This just allows you to plug the existing wiring harness in to the Tenet Box, then hook up the Tenet box to the new speedo. From my research the Magic Box and the BEP 3.0 offer similar capabilities.
Motogadget M Unit - used to replace most parts of the guts of the bike (lunch box under the seat). You can hook up aftermarket speedos to this as well, without the Tenet.
Can you use them together? Yes, I suppose you could, but I'm not sure it would add much value. If you are already doing the work to hook up the M Unit you'll have everything apart and should be able to wire up the aftermarket speedo from there, although you'll have to cut the wires and hook things up terminal by terminal.