Why do Universities offer a "Double PhD Program"? 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?
I think the site where you found the information was pretty crystal clear:


  Joint-PhDs are doctorates which are done at two degree-awarding institutions. This type of doctorate, also called split-site PhDs, does not mean that you are simply supervised by one supervisor in one university with another advising you on an informal basis. Joint-PhDs mean that you are fully registered in two universities, having to comply with admission requirements and assessment regulations at both institutions and that it will result either in two PhD degrees (double PhD) or one jointly-awarded PhD (one diploma with the two university logos). The reason universities offering double degrees rather than jointly-awarded degrees is often practical in nature, for example one countrys [sic] legislation does not allow joint-degrees.


I'm not sure which part of this is unclear. Often it's easier for two institutions to each grant a PhD (i.e., double PhD) than it is for them to create a hybrid entity that would grant a singular degree or for the creation of the institutional apparatus and coordination necessary for a joint degree program.