B1 and B2 can be issued separately but often issued together. The difference is for some countries the limitation of each type is different. For example if you are from a certain country a B2 visa will be limited to 3 months single entry while a B1 can be 1 year multiple. Applicants can request for one specific type depending on the purpose of travel and the duration allowed. The combination of both will always follow the lesser validity.
Yes, a single visa either B1 or B2 can be issued, but often they are combined. According to the official ustraveldocs.com:
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:
As the table shows, they are issued separately but in small 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 is this totally depends on the interviewer at the consulate/embassy.
Credit:stackoverflow.com‘