What is on the disk? Is it a dvd-video? If it is a dvd-video, you can do one of two things. You could compress it slightly to fit a single layer dvd, or you could split it into two parts using either DVDFab (there is a free trial) or you could use an old free program called DVDShrink. You'd need to use DVDShrink in ReAuthor mode.
The last version of DVDShrink was 3.2 and can be found here: http://www.videohelp.com/tools/DVD-Shrink
Here is a guide for an older version of Shrink, showing how to split a dvd movie: http://dvdshrink.info/splitting-v2-3.php This guide is still effective with the newer version. DVDShrink can open an ISO file directly by starting the program, then click Ctrl I to find the ISO file on the computer.
DVDShrink and DVDFab can also compress the movie for you.
If your ISO file is not a dvd-video, but is instead simply files of data, you can mount the ISO in a virtual drive and select whichever files you want to put into a dvd. VirtualClone Drive is free and effective: http://www.slysoft.com/en/virtual-clonedrive.html