Well, you can do a little experiment:
Copy the content of the DVD-9 image to a folder, then split the content into two halfs. What you want to seperate are the .cab files. Copy the first CABs to the first disc along with all the small clutter files, and the remaining cab files copy them to the other disc image.
Now test the installation by mounting the first ISO file, if the installer asks for the game DVD or a specific files (it should be one of the cabs on the other disc it’s looking for), unmount the first image and mount the second image. If it asks again for a disc then it probably needs to access some files on the firt image again, so try if mounting the first image completes the installation.
If everything works out that way, you can write the two images to DVD±Rs. You should really make ISOs files first instead of writing the split game DVD contents directly to discs.