I have never done a problem like this but I will give it a try.
The angle between the x-axis and y=2x is
The bisector of an angle make half this angle with the x-axis, so .55355 radians
tan(.55355) = opp/adj = rise/run = m2 = .6180
So I think the formula for a bisector is y=.6180*x or y=(tan(atan(angle)/2))*x