Yes, a single visa either B1 or B2 can be issued, but often they are combined. According to the official [ustraveldocs.com][1]:
  
> Often, the B-1 and B-2 visas are combined and issued as one visa: the B-1/B-2.

Now, the proof that single B1 or B2 visas are issued in some cases is the official statistics found on [travel.state.gov][2]:

![enter image description here][3]

As the table shows, they are little issued separately but in little numbers. Why they are issued separately in some cases I have no idea and I could not find any information related to this in any official or non-official references, my guess this totally depends on the interviewer at the consulate/embassy.


  [1]: http://www.ustraveldocs.com/sa/sa-niv-typeb1b2.asp
  [2]: http://travel.state.gov/visa/statistics/nivstats/nivstats_4582.html
  [3]: https://i.sstatic.net/8Aev2.png