Today I built one of the test boards I made with the bus multiplex chip and the current Orthrus design (with the XMega32U4), and modified the firmware as appropriate. The result works just fine. This shows that at least the SPI bus can be moved back and forth between two cards in this way. This version was just a test - I ripped the TRNG circuitry off the board, so it's not a full implementation (it wouldn't be able to securely initialize a new pair), but it does mount existing pairs just fine.

The next step will be to complete the new v3 design and get the boards ordered. There's nothing really holding that back. It's just arduous to add all the bypass components the datasheet calls for and route the circuitry. Since it's Labor Day weekend here in the U.S., I think labor will be my calling.