Oh woah. Separate them. 2.5 is a very small area to split. Or at least get some craft mesh and make it so the divider is completely dark so they cannot see each other. It doesn't matter if they're not reacting to it now, but in that area they should not see each other. I know what it's like living on a budget, there's no shame in getting creative! Try a baking bowl or unused vegetable crisper from the fridge or an old paper waste bucket (all cleaned out, of course)
Are the missing fins in clumps? Like little noms taken out of his tail? I would say its fin nipping most likely due in part of being in the split 2.5.
If you need to treat one but not the other for SBD, he needs to be separated anyway.
To treat his SBD, pre mix 1tsp epsom salt in some treated water and really mix it well. When all the crystals are dissolved acclimate him to that. Fasting will help for a day. But keep him on the epsom salts as long as he needs to be. 30-50% daily water changes with more epsom salt water will be necessary depending on the size of the quarentine tank.