As I understands your requirement for a solution: You want a bootable usb stick with multiple linux distros on it.
You can solve this by using grub2, gparted, and dd/unetbootin.
1: format and partition your usb drive to the fit your distro needs
2: install grub2 on your usb device to enable distro selection on boot.
3: copy/install distros into partitions.
I did a similar thing a while back: https://github.com/magnuskiro/grubRescueDisk (I see that the readme needs some work, but at least you should be able to find something useful through the references.)
I have multiple (4 I think) partitions with different linux live images available on a usb stick.
I formate each partition with gparted and used grub2 as a bootmanager to select distro on boot.