Bryan, Double pane window units are just that; a unit. The two panes of glass are held apart by a spreader bar frame, but they are sealed together at the factory with an inert gas between the panes. When you have one that gets condensation or fogging on the inside of the glass panes, the panel has lost it's seal & the entire glass unit must be replaced.
If these are newer windows, many manufacturers will warrant these panels for as much as 10 years, and at least cover the cost of the replacement unit, minus labor. To find out you'll need to know who manufactured the window, and the age of the window. This is normally found stamped in small lettering on the spreader frame between the panes of glass.
As far as replacing the glass unit, this varies greatly with the type of window, and would probably be best left to a glazing repairman.