Make sure that you have marine mix (like Instant Ocean) and not just aquarium salt. You want marine mix to change the SG (specific gravity) of the water, as in contains additional minerals and things that aquarium salt does not.
Your hydrometer shouldn't read below 1.000 -- 1.000 is freshwater, with no salt. It's hard to say how much salt to add to increase by .002. It's not an exact thing, as each mix varies. It will take some experimentation on your part (don't do this to the tank itself and subject the puffers to the stress of changing salinity as you try to get it right -- do it separately, in a bucket or extra container).
I personally wouldn't have two F8 puffers in 72 litres, as that's less than 20 US gallons. The general rule of thumb is 15 gallons for the first F8, and 10 gallons for each additional -- and that's at minimum. I have one Figure 8 puff in 29 US gallons, and she uses every inch of the space.
Congratulations on your new puffers! Just a warning: They're addictive.
