I'm no expert, but if you do decide to introduce a second cat, you might want to try a male cat instead of a second female. We already had Saphirah when we brought Chloe home from the shelter. There were some pretty spectacular fireworks at the beginning, but they finally reached an understanding. We dreaded what would happen when we introduced Samson into our home, but the girls accepted him very quickly. The hissing/spitting fits just didn't happen. The only difference that we could see is that he is a male. Also, when bringing in a new cat, make sure it has its own food dish and litter box. Keep it in a separate part of the house, and introduce him/her gradually to Dallas. Don't expect instant bonding.

Dusty