Double shocks are only used for high speed applications to basically stiffen up the ride and in certain cases to prevent shock fade (shock oil heats up and loses damping). There isn't much sense of double shocks for offroading where you need a softer suspension to roll and climb over obstacles, too stiff would not be helpful especially for slow speeds.*Besides in most cases one can find one stiffer shock valved to the specs of two single ones...
Generally on leaf sprung vehicles double shocks should be avoided unless being used for a specific purpose or specially valved to match the spring rates. Normally the car becomes too stiff to be of any normal/enjoyable use. Coil sprung and IFS vehicles put more stress on shocks and double shocks are more relevant but only for high speeds apps....
On loading vehicles, double shocks could be used and prove useful to some degree, but a bigger stiffer leaf pack is a cheaper and longer lasting alternative which perhaps also works better in many ways.*