They stack if they have different morphs. In my vet dungeon team I (as tank) run Echoing Vigor and the Stam DPS runs Resolving Vigor, they stack.
Edit: if the two people have the same morph (like your example) they wont stack.
If there are three people and one has Echoing, one Resolving, and one UNMORPHED, those stack as well. But rarely do people go that far. If the third dps in my team was stam, we would though.
They stack if they have different morphs. In my vet dungeon team I (as tank) run Echoing Vigor and the Stam DPS runs Resolving Vigor, they stack.
Edit: if the two people have the same morph (like your example) they wont stack.
If there are three people and one has Echoing, one Resolving, and one UNMORPHED, those stack as well. But rarely do people go that far. If the third dps in my team was stam, we would though.
Are you absolutely sure about this?