MultiBootUSB is a free and open source cross-platform application which allows you to create a USB drive with multiple Linux distributions. It also supports uninstalling any distribution at any point in time, so you can reclaim space on your drive for another one.
Download the .deb package
and install it
sudo dpkg -i python3-multibootusb_8.8.0-1_all.deb
The recommended filesystem is FAT32, so make sure to format your USB drive before creating a multi-boot USB stick.
Download the ISO images of Linux distributions you want to install.
Once you have everything, start MultiBootUSB.
The home screen asks you to select the USB disk and the image file for the Linux distribution which you want to put on your USB.
MultiBootUSB supports persistence for Ubuntu, Fedora and Debian distros, which means that changes made to the live version of the Linux distributions are saved to the USB disk.
You can select the persistence size by dragging the slider under MultiBootUSB tab. Persistence gives you an option to save changes to the USB drive in runtime.
Click on Install distro option and proceed with the installation. It will take some time to complete before showing a successful installation message.
You can now see the distribution in the installed section. For another OS, repeat the process.
Next time I boot through the USB, I get the option of choosing either of the distributions.
You can add as many distros as you want and your USB storage allows. For removing a distro, select it from the list and click on Uninstall Distro.