One burner verses two buners will provide double the amount of forced combustion air...this is all good, if draft can remain suffient for both burners.
Any loss/reduction in draft will allow negitive pressure to be diminished to a certain point...and possibly to the point of even becomining a positive pressure.
We all know with positive pressure carbon monoxide will escape into building.
This sure sounds like a draft problem.
I would venture a guess, that with the same wet coal and one burner the condensation was not noticed...because draft was suffient.
Drying the coal would help...but it will not cure a leaking gasket or draft issue. Try reading the "over the fire" draft...that reading is more revelant to hopper condensation than reading draft above the breech.