Select the most appropriate one-word substitution for the given group of words. 

When two things befall together 

Coincident

The most appropriate one-word substitution for "when two things befall together" is coincidental.

• Coupling means the joining of two things.
• Sapling means a young tree.
• Occidental means relating to the west.

The word coincidental comes from the Latin words "co," meaning "together," and "incidere," meaning "to fall upon." To be coincidental means to happen at the same time by chance. This can be used to describe two events that are not related in any way, but happen to occur at the same time.