The DAC page 10 manual says it:
To enable the recording of the SPDIF input via USB go to I/O, Source, and select either USB (Rec opt.) or USB (Rec coax). Record SPDIF optical and Record SPDIF coaxial activate the full duplex operation with USB: The respective SPDIF signal is clock source and can be recorded via USB
But manual page 45 shows the culprit:
22.3 Multi-interface Operation
The current driver supports up to three RME devices of the MADIface series. All units have to be in sync, i.e. have to receive valid digital sync information. Under ASIO all devices are presented as one ASIO device with all available channels as I/Os.
If one of the units is set to clock mode Master, all others have to be set to clock mode Slave, and have to be synced from the master by feeding ADAT, AES or SPDIF. The clock modes of all units have to be set up correctly in their Settings dialog.
In most setups clock sync will become a problem due to lack of enough digital I/Os.
The digital source for the DAC (you want to record) would need to become clock master, and send the same clock to the Pro for sync.
This config would, in total, gain no extra input.
Maybe you’re better off with a Digiface USB, another ADI-2 Pro or 2/4 Pro SE, or even an UCX II.