Okay,
I am sorry this is my second thread in quite a short while, but I honestly am at a loss as to what my actual problem is, so what to look for.

I would like to have a USB-drive with more than one live-system on it - say Debian and RedHat.
Now, I am well aware that it would be far easier to just have two sticks since they are dirt-cheap, but that would teach me nothing about the nature of live-systems or bootloaders, so ... no real use recommending that here. Also I appreciate there is a tool MultiBootUSB, but I would at least like to have a conceptual understanding of what it is most likely to do, and again: how the entire "booting from USB actually works.

But I'm not really sure how to go about it.
The Debian-docs on how to set up the installer on USB-drives seem to suggest that to make the USB-drive "bootable" it needs a bootloader, a kernel-image and some initial RAM-image - which all seems to be magically included when just copying the usual .iso-files as is suggested for convenience.
So I assumed it should be possible to have a bootloader on the device that might distinguish between a variety of live-systems. However, those live-systems to me are a somewhat mysterious black-box, since they just come as .iso-files.

From setting up GRUB for being able to use my old Windows - which I never ever touched since -, I recall that it was rather necessary to have separate partitions for each OS.

So ...
is a bootloader on the USB-drive even necessary for what I'm trying to do?
Is there an obvious easy way that I didn't think of?
If yes and no: which bootloader would I use for this? Is this something GRUB could do, and if not, why not? Is it then even a good idea to go with the live-system .iso-files or would I set up the live-systems differently?

For the sake of your situation. I assume you want to do real installs to this usb drive just like it was a real hard drive. For this I usually say to remove all internal hard drives and be sure the usb shows up as a hard drive choice in bios. (not counting bios type uefi/legacy)

Then I follow the installers. Install the OS to the drive as if it were a hard drive. Grub or some loader will have to allow you to select the OS. Grub could be on almost anyplace the boot could access. I prefer to have grub on my flash drive.
That way I won't have any conflicting issues or changes. The flash drive will be mostly machine independent.

There could be a lot more to your question but I tried to make it brief.

If you want 'live' systems on your usb drive, I would suggest reading the information at the link below which gives some details on the concepts. You could install Grub2 to the flash drive and then simply copy the various iso files of your Linux distributions to the same or a different partition on the usb and use the loopback boot menuentry for each iso. How to do this with examples is shown at the link below. You would need to manually create your grub.cfg file for this to work.

As to having a separate partition for each OS, that would be necessary if you are doing an actual full install but if you are booting a 'live' iso directly with the loopback method, they can all be on the same partition.

https://help.ubuntu.com/community/Grub2/ISOBoot

easy2boot and other things to load live systems from .iso's. I tend to just partition a drive and have full installs on the partitions. With grub in the MBR. I also have multiple sticks. You can chainload grub to the "other" sticks so a hub and couple sticks isn't that much different than just one. Just be sure to use labels or UUIDs so you don't fall victim to /dev/ roulette.

"c"
GRUB> insmod part_gpt
GRUB> insmod ext2
GRUB> ls
GRUB> configfile (hd1,gpt1)/boot/grub/grub.cfg