The main problem with trying this is generally M recommend that people set the centre channel as master. To get the spdif switching on your 7Ks to work you would have to rewire your system so that either Left or Right 7K is the master.
This is not true as far as I recall.
The Comms has to go through the master (perhaps centre) but as long as each speaker is configured correctly (I.e. with AUX using the second SPDIF input on the first of the main stereo pair) there should be no reason to change the master.
Before going too far down this route it might be useful to know where the master is.
(My master is rear right.

)
Ludwig, I think I beg to differ if I understand your comment accurately.
For a DSP to use D1 and D2 inputs it has to be master (not slave). When set as slave the option to define D1 and D2 as inputs does not exist. Only when master is selected does this option appear.
So, if OP wants to test C15 into D1 and MS600 into D2 then it must be set on the master DSP.
If you have this working differently please shout up as I tried a couple of weeks ago (and failed).
Cheers