You DO need to purchase another copy of the PS3 version of the game - this game in particular is locked to the PSN account on which it was purchased / activated - when you even only try to run it on your own PSN account it will fail. Considering you can get the boxed version online dirt cheap (even PSN version are now discounted) and that it will add another 30 free days to your own SW account it's not that much of problem anyway. It will indeed use only one "installation" of the game files on the PS3 not wasting the space / time by downloading it all again, only the game save files will be separated for each PSN user account there.