Ah hell I've been watching this show, it is exactly as you describe
it.  They buy two exact cars, two teams of two guys each do some minor
build-ups on em to hawk somebody or another speed product then haul em
out to the local dragstrip to run head to head.  

The way it works is these brothers host the show and pick up two
identical cars, the one I saw it was IROC Z's.  They bring in the two
teams of two guys who do a coin toss to see who picks which car.  They
then dyno em to get a baseline.  After that both teams are given mods
to make by the hosts, usually designed to Hawk some sponsors product.
If they get those mods done in time they are offered their choice of a
second mod from a choice of like three.  They then dyno loth cars
again to see what improvements were made and head out to the track.
The winning team gets both cars, or in another words each guy on the
team gets to keep one.  The teams are usually made up of guys that are
buds.