[Dunc]

Remember the length of the shock absorber will determine other factors such as droop.

One reason for having longer shock absorbers at the rear of a car is because they are typically mounted further out on the wishbone relative to the front and hence must be able to cope with a larger arc/travel, whilst maintaining comparable droop and total travel. Additionally, mounting further out on the wishbone dictates a taller shock tower if you wish to maintain comparable shock angles front and rear, also leading to a longer shock absorber. Shorter rear shocks would inevitably need mounting further inboard on the wishbone, which may have an undesirable effect (depending on the overall suspension geometry??)

I guess you could try and do some simple spring/damper calculations to see what spring rates and damping coefficients would be required for comparable damping between 1 spring and damper versus 2 springs and dampers. This sort of thing is the basis of dynamic systems analysis and should be easy enough to research if you're that keen. Not sure how you could easily to determine an accurate damping coefficient though....