Reformat and repartition USB Flash Drive From Windows OS

You can do this in DISKPART in Windows:

1. Open an command prompt using Administrator rights.
2. Run DISKPART
3. list disk
4. Note the disk number that corresponds to your USB drive (it should be obvious going by size)
5. select disk X where X is the number from step 4
6. list partition - There should be two, numbered 0 and 1, each about 7 GB
7. select partition 0
8. delete partition
9. select partition 1
10. delete partition
11. create partition primary
12. exit
13. exit
14. In Windows, go to Computer and try to open the disk. It will ask you to format it.
15. Format it with the default settings and give it a name if you want.

It should now a single, unified partitioned drive.

Source: How to delete a partition on a USB drive?