Clams social habits

[ChefFish]

So I recently added a second clam to my tank, both are squamosa clams. When I placed him in my tank there was about 6 inches between him and my other clam. Over the course of the first week or two I noticed that they were almost next to each other. I have an engineer goby (what a pain in the ass!) and thought maybe he was digging in the sand around them and that caused them to shift and get closer. About a week ago I moved them apart again and away from where my goby likes to dig. Within a few days they were right next to each other with their mantles and shells touching. Has anyone else had this happen? Is this normal behavior? I'm ok with them were they are, just found it interesting. I wouldn't have thought they were overly "social" creatures.