Why does He 177 have two nose gun positions?

[Johnny .45]

Hey, I was curious whether anyone had a notion as to why Heinkel bothered to add the MG 81 machine gun firing through the canopy glazing on the He 177, when there was already a perfectly good MG 151 gun in the chin? Seems like the arcs of fire of the chin gun and the top turret would protect the entire area, unless the chin gun is very limited in it's upward angle.
Also, I'm curious as to who controlled what guns. Is the chin gun controlled remotely by the bomb aimer, or by the man in the upper sighting station? Neither makes much sense, as the upper sight station is very limited in it's view forward and downwards, and if the bomb aimer controls the chin gun, who fires the manually-aimed MG 81? Very mysterious, two guns covering the forward arc, but only one man to aim them.