...slaved to the converters is not integration of converters.
With word clock BNC cables and T-connectors you can slave many devices but thats not your demand.
On Mac computers you can run different audio interfaces at the same time.
On Windows computers different audio interfaces are possible but they have to use the same driver.
In your case you would have to find old used stuff.
And in both case, Mac and Win, each interface has it's own Totalmix, no combination.
In the MacOS an aggregate device is possible, combination of all channels to one virtual device for the DAW and the computer…
I would not invest in old equipment, better check for a new system.
A Digiface-USB is cheap, but it has only the same amount of Adat channels as the old Digiface.
If your computer has enough space, a set of two hdsp RayDat would be nice, 6 x Adat.
M1-Sequoia, Madiface Pro, Digiface USB, Babyface silver and blue