How to merge two Mac partitions?
Try to run Disk utility upon system start up, this may increase functionality. So delete second volume and extend  Macintosh HD by dragging it down to make as a single unit. Also repair disk and permissions.

From the screenshot I noticed that you selected your boot disk, which is not possible to edit while OS is running, however second split of HD should be editable. To make less confusion, consider renaming HD, for potential data prevention while modifying HD