In order to see Renee and Toby's rival heart events, you must have the required amount of hearts with the rival who is your same gender for each event.
If you are playing as a female, you must have 2 hearts with Renee. If you are a male character, you must have 2 hearts with Toby.
Walk to the Caramel Falls to trigger the heart event between the two. If you are having troubles triggering this event, try to enter the falls on a Monday, since Renee and Toby are both guaranteed to be there at this time.