In a medium skillet, melt butter and sugar over medium-high heat. Add pineapple and bananas and cook until bubbly, about 2 minutes. Remove from heat; stir in rum.
Scoop 1/3 cup each coffee ice cream and vanilla-fudge ice cream into each of 4 sundae dishes. Spoon warm fruit sauce, and, if desired, candied pecans and whipped cream on top.