As a general rule, anything you do with a copyrighted work in private quite possibly falls under fair use, as long as the copies you are using were obtained leg

As a general rule, anything you do with a copyrighted work in private quite possibly falls under fair use, as long as the copies you are using were obtained legally. If you want to write Shawshank Redemption fan-fiction and read it to your kids in bed, even if Stephen King found out and decided to sue you, you're probably covered under fair use. Creating a program just for yourself that uses two separate copyrighted programs (that you obtained legally!) is almost certainly legal - even if it would be completely illegal to distribute that program to others. This gets murkier if you're using it in a commercial setting, though.