the II and IV chords of a major scale will appear together in every major scale that one of them is a part of.
In the C Major scale, the II is a D minor and the IV is a F major. If a D minor occurs in a major scale (as the III or VI), there will always be a F major in that same scale and vice versa.
I know this has a huge significance, I just don't know what is is yet. I'm only writing this down so I can get it out of my head and onto something. It's not really a question or anything, but if anyone has anything to add to it that could help speed up my comprehension, please, feel free...
Addition: the same goes for the III and V