I'd like to have both live-usb linux distribution and some random data stored on the same usb stick. So that if I plug it in and tell bios to load from it, live partition will be started, and if I just plug it in when some other OS is already running, only the data partition will be visible and will act as a regular usb stick. Is it possible?
Yes, you can do a multiboot USB drive with one or many ISO images inside. If you boot from it, you choose the ISO, but otherwise you can use the stick normally.