It is not possible to use the second ADAT input to achieve 8 additional inputs.
The second ADAT input is used for either double sample rate, or the TOS link S/PDIF format.
Please see the notes below taken from page 8 of the manual:
ADAT/Toslink I/O connectors: the optical connections carry either Toslink (optical S/PDIF) or ADAT signal.
- At normal sample rate, 8 channels of ADAT is carried on a single lightpipe. Use the MAIN connector.
- At double sample rate, 8 channels of ADAT is distributed on 2 lightpipe connectors. Use both the MAIN and the 96 kHz connections.
- Studio Konnekt 48 handles up to 4 input channels and 4 output channels of optical S/PDIF via Toslink.