@Filip ,
are these 2 partitions on your USB drive? Or got the USB drive 2 drive letters?
Even PowerShell is nice, the easiest way is to copy the data from the second partition to the first partition and use Disk Management to remove the second partition and resitze the first one.
Also copying all data from USB drive in a temp folder on your computer and erasing/reformatting the USB drive is an option.
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Regards
Andreas Baumgarten