A 36" tank is more gives enough space for an 6"-8" fish you know. It's just the same as giving a 12" fish a 4' foot tank.
Think about this you are giving your 2 GTs a 4' tank. I assume that 1 will probably reach 12" because I believe that you give them the best possible care, in that case, you are giving him a swimming space of 4x his body length from left to right and 1.5x body length from front to back of aquarium. On the other hand 8" of jd on a 36 would have 4.5x body length of swimming space left-right and 1.5x swimming space front-back. That's actually more relative space that you provide for your GT. And your raising 2 so your actually giving much less space than he will be giving his jd. And if you consider the height of the fish, he will have even more space since jds are more vertically compressed that a GT. You know.