First you said it was a 100gallon tank? Now your saying 2 120 gallon tanks?! Okay, somebodys lying IMO, a bit of proof as in PICTURES would be nice.
A 20 gallon is way too small for even 1 FH, let alone 2! and why do you want to make them agressive? They're beatuiful fish, but you don't deserve them if you think that a 20gallon will be fine!