JPL drives both of them to a halfway point across the planet
(c'mon, you know they'll last that long) where they will meet
and engage in the first BATTLEBOT contest on another world!
Don't tell me this wouldn't increase their visibility (and NASA
and space exploration's) in the general public! And we would
get tons of motorheads and technogeeks wanting to send
their own battlebots to fight it out all over the Solar System,
thus increasing funds for future space missions!
The winner gets the other one's remaining good parts to last
even longer so it can conduct more science to expand human
knowledge about the Cosmos - oh who am I kidding? It's so
the suviving MER can fight MSL! I am not including Phoenix
here because the poor thing can't move so that would not be
fair, but it does have that long mechanical arm which might
make things interesting after all.
Of course MSL is bigger and does have that laser and will be
much fresher than either surviving MER, especially Spirit, but
that will only make the contest even more interesting in a
David and Goliath sort of way.
Okay, okay - the MERs can do a little science along the way
to their final meeting place, but only if they find rocks that
look like little people or faces, because we all know those are
the REAL reasons we sent the rovers to Mars.
Next Up: Dueling Asteroids! Get a bunch of smaller asteroids
like Itokawa, stick a rocket motor on one end, them remotely
smash them up out there in the wastes between Mars and
Jupiter! The one with the least damage - or still has its rockets
working or exposes a really valuable mineral - wins!