How to create bootable ISO from segmented Windows 7 image on El Capitan
Windows 7 (any version) fits on one disk of any optical media, not sure what is going on with what you have or exactly what it is, that said...

You can't boot from an ISO, you need to burn it to CD/DVD or install it to an external drive of some sort in a way that makes it bootable.

Depending on what is going on here you may be able to use Disk Utility.app to create either optical media or a USB drive that is bootable.

Open Disk Utility and select "Scan image for restore" and select the ISO. Attach your USB drive and restore the ISO to that USB drive, which should (I believe, it's been a while since I tried this...) give you a bootable Windows install disk.

Also depending on which Mac you have and the OS version BootCamp assistant may be able to create a bootable Windows 7 installer.