This one is fairly simple and can be gotten for BOTH players as long as one has the DLC required to do it.
Stockpile games are marked by white luminous spots and are far and few. The easiest ones are in the Los Muertos area. Use teamwork to get all the orbs, technically its a challenge to see who can get more, but if it's an achievement you are hunting for; just work together to make sure you get them all in under 2 minutes and 30 seconds. Any of them are easily do-able if you work together on it.