How to merge two volume external USB disk back to one volume?

Make sure View (top-left of Disk Utility window) is set to "Show All Devices".

If you have data you want to save on either volume, copy it to another drive. Open Disk Utility in the Applications/ Utilities Folder. Select Erase and format the drive the way you want it. Then click on Erase in the dropdown menu.

If you just want to delete one volume, select Partition. Highlight the volume you want to delete, click on the Minus sign and then click on Apply. In the dropdown window, click on Partition.

Welcome to Disk Utility on Mac - Apple Support

https://support.apple.com/guide/disk-utility/partition-a-physical-disk-dskutl14027/18.0/mac/10.14

That's strange. It is more than having two volumes, it looks like they are treated as two separate physical drives.

First, of course, you should make sure that anything in this drive is backed up.

You should be able to do this without erasing the whole drive if the volume you want to keep is the first.

Select the whole drive and click Partition.

Then select the second volume in the partition circular diagram and click -

Finally extend the first volume to use the available space.

Isn't there any UNIX-commands that I could use to fix the problem?

No.

You'll need to copy the data from both Volumes onto some other device. Then, use Disk Utility to Erase one of the Toshiba devices. Hopefully, that will erase both and report it as one device.

Then, you can move the files back.