HI all,
I was creating an USB bootable stick whit Debian 9 and having 16GB I was thinking that I could create a multi OS USB stick but I read that some software to do this edit distro files and don't make all ISOs bootable, so..
If I create multi-partition USB stick with different linux distro PC's will read my USB stick like more sticks and I will can boot desidered partition or I need to have only one partition and merge the distros?

There are a few issues here.

One is how you are installing a distro to the usb.
Two is the size of the usb.
Three is the distro choices.

There are two basic ways to install to a usb. One is installing the distro in a normal mode. To modern distro's and hardware, a usb is like any internal drive.
The other way is to use ideas at www.pendrivelinux.com where a "live" distro can be installed (sort of) to the usb. It may help preserve space if you used some of their ways. Speed is also an issue with your choices. A live uses a compressed image and may appear to be faster.

The size of the usb is not excessive. 8G for each distro is about the minimum for a normal install.

Not all distro's easily install to a usb.

For normal installs, the installers usually do a good job. They tend to fix the loader to let you select either. Be sure you remove the internal hard drive when you try this. Installers may put loader on wrong drive otherwise.

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You can use unetbootin to put an iso on a specific partition to have multiple distributions. Might be able to do this with pendrivelinux also, I've never used it so I can't say. The problem with this is you will need to manually configure the original boot menu which code you put it the MBR to add subsequent distributions to it. This will probably be syslinux/isolinux for a boot loader.

You can also install Grub to the MBR of the flash drive pointing to a partition where you have the boot/Grub files. This will also require you to manually create a grub.cfg file which shouldn't be too difficult, just find a basic template online or modify a current grub.cfg. Using Grub2, you can just copy the iso to the flash drive and use a loopback entry to boot it. This works with a number of the major Linux distributions but certainly not with all.

You can also extract the iso by loop mounting it and copying the directories/files to a specific partition on the flash drive and then putting an entry in the grub.cfg file.

You can also do an online search for multi dvd creator as there is some software to do this but it generally only works on a limited number of distributions.

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To do USB I install a "blank" harddrive into a laptop (removing the normal drive), partition it the way I'll want it on the USB and do the install, once installed I switch the new install into an external usb enclosure (and replace the laptop's normal drive), then just image the hard drive (DD) to the usb using the blksize and count operands so the image doesn't overrun the USB. I haven't done a dual-boot system this way but it may work just fine. I've tried to do this installing to a second drive/external drive, but grub doesn't like it and I had to twiddle with settings in grub rescue in order to boot the darned thing...
...as a note, I tried unetbootin not long before I went the way I do it now, darn thing ran about 12 hours and fell flat on its face when I tried to boot what I got.

Good point zOSGuy, reminds me that I've made virtual machines in the sizes of my usb flash drives and just dd'ed over the build to a usb.

I use ez2boot to install multiple OS live-cd and install images to a usb thumbdrive. Several small linux (think DSL, TinyCore, and Puppy), a ReactOS, freedos, kolibriOS, gparted, two AV distributions, and one Windows install image.
I use a 16G drive, and half of it is the Windows ISO.

The install amounts to formatting the USB device, loading the ez2boot install from my laptop to create the USB device properly, after that copy an ISO file into location and run a script. Boot off the device and it offers a menu to boot whatever I want.

Advantage: fast easy, all the options I can fit in the space. Way cool!
Disadvantage: you are running live-cd mode without persistence. Every time you boot is a clean start.

Best use cases: AV and partition fixing on the go, being able to test new distros often and on native iron, and being able to go to install mode and load a machine quickly from the same device.

I am not sure if it would serve your purpose, but if it would this is the easiest solution I have found for a multiboot usb device.