Often tree swallows will use the second box. If located at a greater distance, tree swallows may occupy both boxes. They aren't as territorial as bluebirds.
Sparrows prefer to nest at a higher level.
The house sparrow population will become higher each year if people allow them to nest in bluebird boxes. House sparrows can be a severe problem and cause bluebird nesting failures in numerous boxes, cutting into our bluebird population. If sparrows nest in an occupied bluebird nesting box they will destroy the bluebird eggs or kill the bluebird fledglings, and even kill the female bluebird on the nest. If you wish to help in the recovery of the bluebird, it is essential that you monitor your nesting boxes and control house sparrows. Unlike all native songbirds, house sparrows and starlings are not protected species and you may deal with them anyway you wish.