It's not. At least not in the format you describe without some pretty hefty work going on in the background.
Assuming you're talking about Linux here?
You can boot multiple instances of Linux from a single flash drive, yes - but you have to partition it separately for each OS you want, then install your various distros to your different partitions, then finally set up GRUB to recognise the individual partitions. Just dropping ISOs on the drive will register them on there as files in a file system, not individual partitions.
Of course, there may exist some program that does this all in the background - but I've certainly never seen it and even if there was I'd encourage you to still do it the above way just so you know what you're doing! It shouldn't be that hard, as far as Linux is concerned it's just another mount point.