Guru3D found two references to the Radeon HD 5900 series in a new ATI driver with OpenCL support. There are two possibilities here, either these models are tweaked Radeon HD 5800 GPUs or the company decided to drop the X2 suffix and these models could be the dual-GPU versions of the Radeon HD 5850 and 5870. Time will tell.
It 'could' be the 5870 X2 being positioned under this device ID, but in some drivers these entries already can be found:
"ATI Radeon HD 5900 Series" = ati2mtag_Evergreen, PCIVEN_1002&DEV_689C
"ATI Radeon HD 5900 Series " = ati2mtag_Evergreen, PCIVEN_1002&DEV_689D