I just scrolled through the student list of my university and found a person who is a “double PhD candidate”. Since I have never heard of that, I tried to find some information. The best link that I found is “Joint-PhDs: A Guide” at Find a PhD.
However, the only explanation given is that,
[...] universities offering double degrees rather than jointly-awarded degrees is often practical in nature, for example one country’s legislation does not allow joint-degrees.
I expected something like research in two research areas by one person. But giving two PhDs because of legislation seems odd to me. Can someone tell me more about this?