I have been able to keep dwarf puffers in the last 6 months and this is my experience: the fact that one would only eat snails and the other would only eat the other type of food is just to show that each one of them has their own personality. with that been said, if you want to feed them snails only you can start by starving them for 1 or 2 days. Then to induce the puffer that normally doesn't eat snails, put him in a breeder box and then add some live snails. As soon as he/she gets comfortable in the breeder box, you crush one or two of the live snails and if you time it right, you will pick its interest in the gooey fleshy "meat". When i say "time it right", i mean the fact that you need to let him/her starve for a proper amount of time. As for the other puffer that only eats snails and nothing else. Do the same thing but this time use frozen bloodworm or live blackworm or whatever is it that you want to feed it with. Dwarf puffers will start to eat shrimps when they're babies. They will not touch shrimps that are as big as they are unless you have a small community of dwarf puffers with at least 2,3 males.