Double joists separating on outside edges of deck
From your pictures I would say the outer one is a perimeter board. These are for looks to dress up the outside of the deck.

The inner board is carrying the end of the deck boards but is not attached to the ledger board with a hanger. Bad technique, in my opinion. Notice, the inner joists are attached to the ledger board with joist hangers. But the outside joist is not. 

You should be able to add more screws to the perimeter board to pull it back in to the outside joist.

Good luck!