Diablo – Marked For Death – everyone or just me

This proc does not refer to which weapon is equipped, rather it refers to the type of skill you are using.

I used this passive while having a crossbow equipped and it would still proc. Also, it procs on any skill that requires "firing your weapon", which are all skills in the Primary, Secondary, and Archery categories.

They do not stack as you will only ever see one buff of that type on the target.

Drew data from this question and links. Do damage over time effects stack?

Since it is a debuff on the target you will only see one Marked for Death on the target, a new cast or other cast will simply reset the timer. If there is similiar type buff then it would be a different debuff on the target and should stack.

Edit: To answer the additional edited question, if we refer to this question then one can assume that they do stack because of the different runes. However, this is a debuff casted on a target, not a buff for the party. So I believe the answer is still they will not stack, but whoever cast the most recent one will have that effect. I don't know another demon hunter to test with at the moment, but I will update once I do.