boot - Create bootable Ubuntu USB on a partition, instead of overwriting the entire USB drive

I have a 64 GB USB pendrive with two partitions, of approximately 10 GB (the first) and 54 GB.

I'm on macOS, and I know how to create a bootable USB drive by using diskutil erasedisk and then dd to copy the latest Ubuntu installer .iso image to the USB device. This removes any previously existing partitions and then sets up the entire UBS drive as one single bootable volume.

Is there also a way to keep the 2nd partition unharmed, and just only the first 10GB partition to set up the Ubuntu installer, in a way so that the device still becomes bootable?

I can also remove the 10GB partition and create a smaller one or something else in its place, if that's required. But I just would like to keep the 2nd partition (the 54GB one) as-is.