I'd advise 1 lever per side. If you have a good set-up it's easy to generate enough braking to get the rear very light and a rear brake can be anywhere between exciting and dangerous under such a scenario. If you are not having a rear brake, except maybe a parking brake activated from a friction shifter, then it makes sense to use both hands to brake the front. Dual pull levers can be quite heavy to use as you are doing the work of two hands.
As you get familiar with the braking and trike it can be useful to be able to put more or less braking to one side to tighten the line mid corner if you ever ride near the edge of what the machine can achieve.
The 3 warriors made have a dual pull on the left for the front brakes. I never thought about it being twice the work like mentioned above, but i can still pull it with one or two fingers. I put a Vbrake on the rear wheel and it stops the trike really well, it also makes power sliding possible.