Opendoor, I think I see a rationale behind summing / averaging the inputs by two SS: avoid potential over controlling (and possible damage) to the aircraft if the two pilots are both trying to make an input at the same time.
For example, if you have a 2.75 g limit, and the Ground Prox warning goes off, both pilots (even though one is flying at the time) may instinctively grab the stick and pull to the limit. If you allowed both inputs to add, then you have a 5.5 g pull. That may be a few g's more than needed, and potentially damaging, and might also induce an accelerated stall just when you cannot afford one. What needs to be done and trained to deal with the situation you cite is one pilot takes controls (I have the controls!) and the other pilot relenquished (you have the controls!) and ONE flies. It seems to me that the control logic is built to accomodate sound CRM principles.
Granted, not pulling
enough could profoundly damage the aircraft via ground impact ... but I think the limit design tries to account for that.
(Aside: There was some very good and informed discussion of this in the Islamabad crash thread where they hit that mountain, over a year ago).
While the situation I cite may not be the best example, I think it illustrates the problem of two FBW inputs being used at once, inadvertently or not, without a filter to process two inputs when, ideally, the system is only ever supposed to have one man flying in the first place.
This follows good CRM practices, usually, except when you need one guy to save us all from the idiot trying to kill us in the other seat ...
).