Procs only affect your character. Also, your pet build usually will have non-existent OA, because you don’t need it at all.
No. This is one instance. The strongest buff will be applied, just like normal buffs with multiple players “stack” (strongest is applied, weaker is ignored). That would be kind of ridiculous if you could stack those. Imagine having Biarthorn’s buff twice. 24% free physical resistance.