For Navi, the left cat, we call her Chunka Munk, Plasticat, and Ms. Infected Lips.
For Scorpi, the cat on the right, we call him Mr. Small, Spider Cat, and Crybaby.
They're different sizes for starters, William is a bit beefier and Edgar is slimmer. Also their fur is a different consistency, Edgar is slicker and more shiny. On top of that, their meows are so different and Edgar is extremely chatty. Normally the only time we get them confused is from a distance or late at night when we're shouting at whoever is being bad.