And I will use logic, and thus go wrong with confidence.

1000kg car weight
20m/s
2 cars.
f=mv^2
f=1000kg * 20m/s * 20m/s
f=4000
00kgm^2/s^2, or 400000joules
x2 = 800000joules energy to disipate in the dual car crash

1000kg car weight
40m/s
f=1000kg * 40m/s * 40m/s
f=1600000joules to disipate in the single car crash

Not responsible for advice not taken...