You would have to combine the ISO's with a program such as DVD S****k in reauthor mode and output to ISO image file or Hard Disc Folder and then manually burn with Imgburn, follow the appropriate guide. Select the best position of your layer break and burn.
It would be much cheaper however, to simply put both ISO's on a SL disc for about $.30 a pop vs. $2.00+ each for the DL media.