If you can see the actual junction, the smaller line, 1 1/2 inch is grey water. (Depending on your layout, there may be two grey tanks that are Siamese'd, or may have separate pulls. (Usually hidden behind front axle.) My suggestion is to dump and super flush both when possible legally. Then visit the dollar store for some cheap food coloring. After dumping and rinsing both tanks, add food coloring to only your kitchen sink, and your bathroom sink or shower, add water, and pull what you think is the "right" handle or handles. Colored water tells the tale! Label tank pulls so they are obvious to others.
Steve & Jan, Ava & Emma (Mini Schnauzers):
2016 F350 Lariat 4X4 Powerstroke CC/SB "PULLTOY V"
2013 Alpine 3535RE "MAGIC CARPET IV"
Proud Navy Vet!