When having doubt, ask

Boss A:  ask him about his preferred communication style - does he want to copied on everything? Frequency of communication?  Detail or high level summary?  written report or skype meeting.  if he does not have preference, try to schedule skype coz u need face to face time.   
Boss B:  help as much as u could.  He might be your boss one day.  But if u are overloaded, explain and give him an estimated deadline.  At the same time, u can ask boss A whether he is ok for u to take B's assignment and ask for advice.