Newb question...washer outlet box drains

Setup...currently have a washer outlet box with center supply and two drains, one on each side of the center supply. However, builder only hooked up left side drain down to p-trap and into drain/vent stack. The right side drain does not have a pipe hooked up to it.

I am putting in a softener in this area and want to run the softener drain into this outlet box using the drain side that currently does not have a pipe running to it.

Question...should I (1) connect a pipe to this drain and then tee into the left side drain above the current p-trap, or (2) connect pipe to right drain and plumb in its own p-trap into waste and vent stack?

I am thinking (1) is right. Just wanted to get the experts opinion before I screwed something up!

drain

You find it easier to make the connection using a "Y" in the drain riser and a 45 degree elbow to line up with the second opening.