I had to use Bridge to associate with CS6, I had to browse tot he actual .exe path (C:\Program Files\Adobe\Adobe Illustrator CS6 (64 Bit)\Support Files\Contents\Windows\ for me).
Now I have both on my machine, but now a double click opens in CS6 (not CC).
My question was "How can I force Windows 7 to open .ai (and .eps) files in Adobe Illustrator CS6 and not in Adobe Illustrator CC".