Typically, one of the elections is by first-past-the-post, runoff voting or bloc voting, while the other election uses some form of proportional representation.
One of the justifications is often to ensure an element of district representation, while enabling minorities to have some element of representation even if it is not strictly proportional. This lack of overall proportionality means that parallel voting is more likely to produce a decisive result than mixed member proportional voting.
It has been used in many countries, including Albania, Armenia, Azerbaijan, the Cameroon, Croatia, Georgia, Guatemala, Guinea, Japan, South Korea, Lithuania, Niger, Russia, Senegal, the Seychelles, Somalia, and Tunisia.