The main mission this time is to learn how water shaped Mars
from direct examination of the surface.
The three orbiters since 1998 (including the new Euro one) have had
high enough resolution to see many water-related structures on Mars.
These include channels and layering. The Vikings were not specifically
designed for water evidence. The pathfinder was an engineering prototype
to test new landing and mobility technology. The new rovers add more
useful science instruments and greater travel range (once they work!!!).

Two rovers at the same time allow (1) save on engineering design costs,
(2) hope at least one works, since mars missions have a 2/3rds failure rate.
(3) Because they are on opposite sides of the planet, you can be running
one in the sunlight, while the other is sleeping, thereby leveraging
communication and earth facility overhead.