Twin shocks are usually fitted to cure shock fading due to high speed use over rough ground. I used to look after a comp safari racer that had twin shocks all round. (racer now sold by owner BTW)
To be honest in most normal off road applications you are better off looking at your spring poundage if you are experiencing shock fade.
The proper solution to shock fade in speed applications is to use a single tunable shock with a remote reservoir. The reservior increases the capacity of the shock absorber without hugely increasing the diameter of the shock body.