The easiest solution is use a DVD+/-R DL or a BD-R.
If this ISO is bootable, and as a Windows install disc it should be, then spanning across multiple discs probably won't work. The installer will look for all files in one location to install the OS.
In fact, as far as I know, ONLY using a double layer recordable DVD or a recordable Blu-Ray disc will work in this case.
Theoretically, if you wanted to span the ISO across multiple discs, you could create one DVD+-R with the boot image from the ISO, copy as many files from it to that bootable disc you're going to make as possible, and burn it. Then, create a 2nd disc with the rest of the files. However, this is very difficult. I've yet to get the hang of setting the various boot image settings in ImgBurn to correctly create my own OS install discs. And, as I said, I would think it's a waste of time as the install application would look for all the files across one disc.
If you're not interested in this disc booting, you could just create 2 DVD's yourself, copying as many files as you can fit onto the first and then the remaining files onto the second. However, you'd be swapping out DVD's at least once during install, IF it even works at all.
As I said, the easiest solution is use a double layer recordable DVD or a recordable Blu-Ray.