Answer
Assign priorities to the groups around top central carbon: OH>CHOHCH2OH>CH2OH>H. This gives R configuration. Assign priorities to the groups around bottom central carbon: OH>CHOHCH2OH>CH2OH>H. This gives S configuration.
Work Step by Step
The 4th group H's are in the front, so configurations need to be reversed. The top carbon is S while the bottom carbon is R.