Let me re-answer, a young Yellow and young Blue would likely be fine together in a 90 for 1-2 years even, but as they get larger/older, territory will likely be more of an issue......They may be fine as adults too, it's not a bad combo, a lot of people will frown on Tangs in a four foot tank, but I would say if you don't overcrowd it might work out fine.....Keep in mind, Tangs need really high quality water, lots of greens in their diet and good water flow, if you can provide that I would say go for it.