Removing one of the triggers is definitely a better equation for your current tank, but the fish will grow pretty quickly. Rather than remove anyone (if they are getting along), maybe look into getting them into a larger set-up within a year or so. I started with a 2" puffer and 3" lion a year ago, and now the puffer is pushing 8" and the lion is around 7" and moving into a 210 gallon tank within a few months. Not that I am saying get that large of a tank, but 2 triggers and 2 puffers need more space than a 75 to grow up into.