The sending of the first probe was announced only a fortnight ago.
Nasa spokesman Dr Edward Weiler said the two robot vehicles would be sent to different locations on the planet's surface to broaden the agency's objectives.
"To have two rovers driving over dramatically different regions of Mars at the same time - to be able to drive over and see what's on the other side of the hill - it's an incredibly exciting idea," said Dr Weiler.
Doubling the number of Nasa probes is believed to be adding about $150m on to the $350m price tag of a single mission, excluding launch and operations.
Costs leap
Officials say the extra cost was weighed against the scientific appeal of running two missions side-by-side.
The sending of the first probe was announced only a fortnight ago.
The Mars rovers will be a larger and more sophisticated version of the tiny Pathfinder Sojourner vehicle, which successfully landed on Mars in 1997 and sent back thousands of photographs.
The probes are capable of travelling 100 metres (325 feet) a day on the Martian surface.
Last year, Nasa lost two Mars probes - the Mars Polar Lander and an orbiting vehicle - when they disappeared on their final approach to the planet.
Embarassment
The incidents were a great embarrassment to Nasa - particularly as it emerged that the orbiter crashed because the mission engineers had confused metric with English measurements.
The new landing probes will send colour and infra-red images back to Earth, and will be equipped with tools and instruments to search for evidence of liquid water which scientists believe once existed in abundance on Mars.
"Our teams concluded that we can successfully develop and launch these identical packages to the Red Planet," Scott Hubbard, Nasa's Mars programme director, said.
The landing sites have yet to be selected, but Nasa says locations might be chosen where water is most likely to have existed.
Mars is thought to have once been rich with water and its surface bears features that apparently were carved by water.
The 136-kilogram (300-pound) landers will be launched on their seven-and-a-half-month journey to Mars on top of Delta II rockets. One is scheduled for launch on 22 May, 2003, the second on 4 June.