Why are there 2 copies of RNA in the HIV virion?
You are not totally correct because these two strands don't have to be the same, they can be genotypically different, which occurs when a cell is infected by two distinct HIV strains. Also HIV uses reverse transcriptase which can "jump" from one strand to the other so sometimes pieces get repeated, skipped etc. So you will get a recombinant --> more genetic diversity(see this article) I won't go into to much detail but if you want to know more I would highly recommend to read the article I linked after genetic diversity. So to summarize:


genetic diversity, because of switching between the two strands (this makes it so hard to cure AIDS e.g.)
If one RNA strand gets damaged it can use parts of the other RNA strand as template, so to synthesize DNA
apparently the dimeric form of RNA in HIV is essential for successive replication of the virus


Source: previous mentioned article(HIV-1 RNA dimerization: It takes two to tango.)