I don't think Andy used ALL 12 strings, I thing he strung it up using only 8 strings, as there weren't too many mandolas around in the 60's to choose from. I think he tuned it gdad, as he tuned his mandolin. Maybe he tuned it like his sobell mandola, as it was two frets longer he tuned tuned it two frets up from the mandolin and capoed alot !

He also used the waldzither, usually 9 strings but never strung up the 9th !

Pictures here and more info here
portuguese mandola