Измените ваш запрос на:
SELECT a.Username, b.StoryName
FROM [dbo].[UserTable] as A
INNER JOIN [dbo].[StoryTable] as b ON a.UserID = b.UserID
WHERE b.InsertedDate =
(SELECT MAX(InsertedDate) FROM [StoryTable] AS z WHERE z.UserID = A.UserID)
Отредактировано согласно комментарию:
SELECT a.Username, b.StoryName
FROM [dbo].[UserTable] as A
INNER JOIN [dbo].[StoryTable] as b ON a.UserID = b.UserID
WHERE b.StoryID =
(SELECT MAX(z.StoryID) FROM [StoryTable] AS z WHERE z.UserID = A.UserID)