I don't know that fitting twin shocks is really a great plan. Firstly, if you are overloaded, it's better to ditch kit - heavier shock will just transfer the wieght problem to some other component, eg axles, mountings, chassis.
On the other hand, if you are just looking for better on- and off-road handling, then I would say you'd be better off with a single pair of good gas shocks. FWIW, I have been impressed with the Bilsteins I fitted.
Also, to help reduce sway, I fitted airbags into the rear springs which have been an excellent investment (about 80 quid an axle, and takes an hour or two to fit a pair).
They can be inflated to compensate for load, and deflated when you want full axle articulation. They need about a 3 Bar air supply (only low volume), but can be pumped with a hand pump or at a garage if you don't have a compressor.
I don't remember any links to Internet sites, but I have posted a link on the HUBB before, so a quick search should find one.