Maltase is an enzyme that breaks down maltose. If a maltase enzyme has just completed catalyzing the decomposition of maltose, then is it still available to catalyze the same reaction with another maltose molecule

Yes it is available. It will continue catalyzing the reactions until it becomes completely consumed. That's how enzymes work. They work and are eventually consumed in the process completely without altering the reaction in any way other than speeding it up.