Brackish is easy. Add some marine salt - use a spectrometer - , cycle, add some fish.
A school of monos would be boss, or archers. A tank that size could also make a great brackish community tank. I have wasp fish, blennies, molllies, orange and natural morphology chromides, bb gobies and spotted ctenopoma in my 180.
If both tanks end up with the same water parameters, a water bridge made of acrylic connecting them would be cool. A guy on reefcentral had a great thread with a movie of his tang traveling through it. Youd have to make the sumps connect, though, to avoid imbalance and a flood.