I run windows 10 and windows 7 in the same motherboard, but never simultaneously. The systems are housed in different hard drives and I choose them through the bios startup sequence.
When I run windows 10, the windows 7 partition appears in windows explorer as drive B: When I run windows 7, it becomes letter C:
As such, all partitions - and I have several for pictures, music, etc - have different letters depending on which system is running at that moment. But I can reach any partition, whichever the windows version I am using. They just have different drive letters.
I admit I did not try to use the same letter whatever system I using; it may not be possible because the partition system sees them all at the same time and does not allow the usage of the same letter.
Ccleaner is bugged by this kind of organization. When running Windows 10, ccleaner recommends cleaning all register records that mentions an inexistent partition H, for example
However, that is precisely the letter chosen as the music partition when I run windows 7. In windows 10 that partition is W:
So that means that Windows keeps the double operating system all in the same registry.
So does that kill the advantage of using ccleaner?