Essentially, the 1st show is for the folks with the first dinner seating, the 2nd show for the folks with the 2nd dinner seating. Yes, shows do overlap seatings, so if you are eating at the 2nd dinner, the 1st show will be happening during your dinner.
No, you cannot reserve seats for any of the shows.
CarnivalSensation February 2015
Carnival Dream November 2015
Carnival Fascination April 2014
Carnival Elation March 2011
Carnival Imagination Sept 2007
Carnival Sensation Dec 2006
RCI Sovereign of the Seas Sept 2006
Carnival Miracle Sept 2005
Carnival Glory Sept 2004
Carnival Fantasy Jan 2004