Another irritating and concerning thing at FL100, when both pulled on the SS simultaneously, I have the bad feeling that they saw the ocean coming close. I think that was just a last try to save something without too much hope.
Back to the technical stuff.
AFAIK there is only one SS active at a time. You have to push a button to take over.
Is Alternate Law 2 allowing dual SS input or were they eventually already degraded to mechanical law?