In order to enjoy maximum ease of use from Sigma it is necessary to connect 32 outputs to Sigma (four dsub connectors & cables). Anything less than that will have operational caveats, explained here.
For instance, connecting 32 outputs (such as using two Alpha-Link MX 4-16), allow each Sigma channel to be switched to mono or stereo at any time with no repatching necessary.
We are aware that in some cases a full complement of 32 outputs may not be available. If only 16 outputs (two connectors) are available via standard db25, Sigma will only be able to handle 8 signals (which may be mono or stereo).
In order to feed 16 mono signals into Sigma, it is necessary to use all four db25 input connectors using custom pinned cables.
So to follow up with the Alpha-Link MX 4-16 example, one db25 output on the Alpha-Link would need to be pinned across two db25 inputs on the Sigma.
So the best solution for this kind of configuration would be to use a patchbay:
Connect all the DAW outputs and Sigma inputs to the patchbay, then manage the connections with patch cables to handle stereo pairs or mono signals as required by the project you are working on.