I also probably wouldn't push more than 2 in a 90. I have a 3 foot tank and my blue spotted will launch from his burrow dead center all the way to the sides of the tank, so I'd say ideally you have at least three feet between them. For what it's worth I definitely would not add a second in my 65, but you might get away with it. I'd add both at the same time though, hopefully to limit squabbles.
And make sure that your tanks is completely sealed, they will find any gap that they can fit their heads through. I thought mine was sealed and turns out a corner was slightly open. Finding your favorite fish dried out on top of the netting is not a great way to start a day :/