Creating 2 ISO files on one flash drive

Hi,
My question is, if I have 2 ISO files for 2 different operating systems, can I create a multi-boot USB? I have Windows 7 and Windows 8 ISO files and I want to format my computer. Is it possible to put the files on one drive and make the drive bootable? What program can I use and what are the step by step instructions to achieve this?
