hermit crab question?

[nano-man]

This is my first time at this so take it esay on me.I've got a 8gallon sphere(looks like a gold fish bowl),Fluval1, desk lamp with CF,4.5lbs live rock.As for live stock,clown,nocternal goby,blue and black stripe legged hermits,2trubo snails,lipstick conch,emerald crab, striped mushroom,green star polyp.Every 4to6 weeks,i find one blue legged crab dead,BUT i still haven't lost the first one.Could two crabs share one shell?