Diamond sleeper gobies (Valenciennea paullaris) are very nice and should always be kept in pairs. The male is usually smaller with a taller dorsal fin. If you want to add more of them, then go with some diversity and add my favourite of the Valenciennea genus, V. Wardi, Tiger Sleeper Goby.
http://www.tjorvar.is/assets/images/Tiger_goby.jpg
Tiger Sleeper gobies stay close to the substrate with their mouths of sand so they don't make a mess like V. strigosus, Goldhead Sleeper Goby. They are also more hardy than the other members of the genus. I used to keep them with their commensal (symbiotic buddy) pistol (Alpheus sp.) shrimp, until I found out that they eat cleaner shrimp and the occasional fish. The clicking noise also drives me crazy as they sound identical to the more fearsome mantis shrimp. It's a cool display with the two gobies keeping watch and the blind shrimp maintaining the cave they all share. The shrimp uses a tentacle to constantly read the signals the "look-out" gobies give. Speaking of which, signal gobies (Signigobius biocellatus) are another favourite of mine that have a three-way relationship with a shrimp.
http://www.starfish.ch/photos/fishes-Fische/gobies-Grundeln/Signigobius-biocellatus2.jpg
and how can we forget our favourite odd couple roommate Randal... not Tony... this guy
http://www.frontiernet.net/~maroasis/randalls_shrimp_goby.jpg
and the shy but resplendent Antenna gobies (Stonogobiops sp.)
http://reeftools.com/wp-content/uploads/2010/03/Stonogobiops-yasha.jpg
http://www.stockphotopro.com/photo-thumbs-2/AKR7T1.jpg
and my favourite invisible fish the cave goby
http://www.liveaquaria.com/images/categories/product/p-90167-Cave-transparent-goby.jpg
The nicest goby, although not a sand sifter, I have had came in by fluke as a "waif" from The Philippines, the Helfrichi Fire Goby
http://gobyfrontiers.org/gf/eg-013.htm
The experts all agree that deep sand beds should be left alone, so sleeper gobies and any other sand sifting fish and invertebrates should be limited to tanks with a maximum sand depth of 2". A remote sand bed can be easily added with a 55 gallon drum on a flow-through somewhere in your system that is free of detritus.