Im not sure there is an exact number to tell you. I had three and am down to one because two got picked on to death, though not sure by what. 2 may even work for you but I think I would probably do 3-4.
3-4 is the lowest amount together i would keep to keep agression down. If you just keep 2 the bigger one will force the smaller dat to a corner of the tank so it can hog the food which will cause him to grow slower. The more room in a tank for 2 dats helps alittle too. I would get 2 more dats if i were you.
i have 6 in a 125 gal all between 1.5 and 3 inch doing great some chasing but nothing major id say atleast 5 im thinking of getting 4 more but not sure as i will already have to upgrade within the next few years when they start to get to the duble digits in size thats if they growth slows down i got them at about 3/4 of an inch about 3 months ago and some are growing like crazy they say they slow growers but mine seem to be getting big in no time at all