I just recently posted a tutorial where I used two separate kits of a blog train to create a design and was asked "Is there and easier way instead of going back and forth between two kits?" Of course.
I download my two kits and save them in a separate folder (called "Supplies") in my design folder.
I save the original files but then copy and paste both files into one folder title "Combine". As I go through my tutorial, I can easily name what I am using because the file names are NOT changed at all!
Once I have completed my tutorial, I simply delete the combine folder and keep the original ones safely with the tutorial!