Drex what are you saying man, are you talking about literally closing the space or just making it appear that it's closed up.
literally you cannot do anything about it, that's your genetics and that's how you were born.
when Drex says bulking up he meant (i hope) that the bulk you put on your pectorials will make it look like your chest has less of a separation.
and for the second question:
well you can either do dumbell presses or you can do dumbell flies. but if you want bulk then the dumbells better be heavy.