That is an extremely pointed and guided question.

I'll treat this like a mini-oral board (since this is the exact type of question you get):

Officer A should immediately notify his direct supervisor, at which time his direct supervisor will instruct him to type up an incident report. Officer B will be transported to whatever type of IA exists in that department for immediate questioning. The park will be held as a crime scene and the blood analyzed to see if they can find the young man. A survey of reports from that night and a week before will be done to ascertain if that young black man had been stopped before, if he had, the supervisor or IA, will contact him and ask him to come in to make a complete statement. Officer B will be removed from active duty and either desked or suspended (with pay) pending the outcome of the investigation. All relevant paperwork to this end will be submitted to the local DA's office for consideration of charges that may be filed against Officer B.

Officer A is also not completely in the right here since he allowed, what is worded to be, an assault to take place. He will also be desked or suspended (with pay) pending the outcome of the same investigation and all that relevant paperwork will also be submitted to the local DA's office for consideration of charges against Officer A. Technically, whatever Officer B did, he did so because Officer A did not stop him and since he did not automatically report the incident to the supervisor Officer A may be guilty of conspiracy and charged with the same crimes Officer B is charged with (if any charges get filed against them)

During this time their direct supervisor will go back through both Officer A's and Officer B's history to see if anything like this has occurred before and what actions were taken. This supervisor may contact previous supervisors of these two officers and see if there was an issue that was overlooked or if they had previous issues relating to this type of action. How many times were the officers disciplined? How many times did they get complaints? Factoring all of that into his report the supervisor will complete his own report and submit it to his commanding officer (who will be doing his own investigation as to why an officer was allowed to be on duty if he was having issues - investigating their direct supervisor).

There will be employee assistance program info blips read out to the officers during their roll calls and given to them in their mailboxes, not just in those two officer's squad but in the entire station (since it may stem from another incident or officer).

This is the ultra-textbook answer you would give during an oral board, there is much more but I skimmed the surface to give you an understanding on how "by the book" an answer to that question can be.