Okay, just to be absolutely clear here...
Let's say one of the movies you have saved to your hard drive is 90 minutes long. When you're finished, you want to have two DVD discs; on the first disc, you'll have the first forty five minutes of the movie and on the second disc, you'll have the last forty five minutes of the movie. And both of these discs you'd like to be able to play in a standard DVD player attached to a TV, correct?
If that's what you want to do, it's fairly easy. However, you can't split an .ISO file directly. If you just wanted to store the file, you could break it into multiple parts with a program like WinRar. However, if you want to end up with two DVD movie discs, you'll need to go this route:
Get a program called Virtual Daemon. You will use it to mount the ISO file. That means that your computer will see the ISO file as if it were a real DVD in a DVD drive on your computer. Then, you can either use DVD Shrink or DVD Fab (both programs should be available for free) to split the movie into two parts. You then burn one part to each disc.
Contrary to what b1tr0t says, asking this question is not illegal. What you're proposing to do might be, but as far as I know no one has ever gotten busted or sued for making a backup copy of a movie for themselves. It's when you start distributing it to other people that you sometimes get into hot water.
And contrary to what soma lkzx says, it's not necessary to compress the movie. I almost never compress mine. Compression is an option and it's often seen as the cleaner, less bothersome approach, but it's not the only route. In any case, if you do decide to compress, DVD Shrink will handle that job nicely.