Can a 240V circuit be split to make two 120V circuits when 240V is no longer needed?
Short answer - you can easily convert to a single 120V circuit but you can't convert to two circuits, you have two wires and you need three or four.  

Your 240V branch circuit is supplied by two wires originating at a two pole breaker in the panel.  You could remove the two pole breaker, install a single pole breaker, land one wire on the breaker and the other on the neutral bar, and you have a 120V circuit.  

To get two 120V circuits, you have two options:  


two separate circuits wired with four wires 
a single multiwire branch circuit with three wires - two hots and a shared neutral  


If you're wired with conduit and have the space, it's usually pretty easy to add one or two wires from the breaker to the outlets.  If you're wired with cable - either nonmetallic / romex or armored MC / AC / BX, it's more of a project.  But it's not a simple conversion.