We all heard the rumors that there's a dual SIM variant of Lumia 720 is in the works. Since Lumia 525 is now out there with 1 GB of RAM, I seriously doubt that Nokia will release the dual SIM varant with 512 MB of RAM. Also during testing, Nokia did put 1 GB of RAM in Lumia 720. So its highly unlikely the variant will ship with 512 MB of RAM, if it still does that'd be very stupid of Nokia.
PS: By the way, limited 8 GB of internal memory bugs me more than 512 MB of RAM. I hope we get the option to install apps on MMC in Windows Phone 8.1 update.