My first impulse was "typcial Guy answer - so short" , but he is right - a double album has not automatically to be a concept album - it is just an album that provides music from the same artistic background/idea/intention - and when 79 minutes on a CD offer not enough space - well, then you have to put in onto 2 CDS. So it is one ALBUM in the artistic meaning of the word, but physically it is two CDs. Hope this makes sense...
LE