Yes it's possible if both CPU are the same generation. E5 v1 with E5 v2 doesn't work.
I d'ont know but I think it's not a problem because when all the core are full the turbo frequency of the 2696v2 goes to the level of the 2695v2.
In the end it's as if I had two 2695v2 in all the benchmark that uses all the core at 100%.