EDITED: contained too many typos and I performed some checks in terms of routing on the ADI-2 Pro.

I regard this not as a valid test, both DACs should get the unaltered audio material digitally
so that each of the DACs performs then only the D/A conversion.
And your HiFi Amp needs then 2 x analog stereo inputs.

If also the other DAC offers an optical SPDIF inputs and if you perform such tests more often,
then I would recommend you to get also the RME Digiface USB recording interface.

The Digiface USB supports four ADAT I/O, where each of those ports can be switched to optical SPDIF as required.
This is a very unique feature for this interface.

Using TotalMix FX you can then easily store the different routings to the different DACs into Snapshots.
Up to four DACs would be possible, its only limited in terms of how many analog inputs are free on your HiFi Amp.

You can even integrate the ARC USB into this setup, then you could switch very comfortably betwee the 4 routing/devices.

Alternatively you can also use a remote control of your HiFi to switch between the analog input channels.

I tried today on my ADI-2 Pro to find a setting, to be able to switch between AN 1/2 and SPDIF out,
but couldn't find a setting that allows for this.

So if RME has no idea how this could be achieved, to switch between AN1/2 and SPDIF OUT on the ADI-2 Pro,
then I think you require something like the Digiface USB additionally to perform such A/B comparisons between two DACs,
assumed your 2nd DAC in the test has an optical SPDIF input.