SPDIF is a PCM digital signal that carries a two channels of audio over a single digital cable.
UC Surface shows two independent channels of control for SPDIF as:
- SPDIF 1 is renamed SPDIF (L)
- SPDIF 2 is renamed SPDIF (R)
You can easily select SPDF 1 and select the Link button to combine SPDIF 1 and 2 into a single stereo channel.
To unlink the channels, click the "Link" button again so it has a black background and your channels will be now separated again.