When hunting on water the decoys are one spread but broken into groups by species. We will have Mallards in a group, with teal on edge of them. A couple blacks off to the side. If woodies are in area we will put a couple pair of decoys around the edge of water near shore.
When you watch birds fly in, either in a pair or small group they will land and swim together. They will swim around other bird types, but usually stay in a little group. I try to copy this in my spread.