Add the options from the Option toolbar the Line from Angle tool to the Options toolbar of Line from 2 Points.

As long the user does not set a value for Angle or Length the tool acts just like Line from 2 Points. During this time the values are automatically filled in corresponding to the line preview on canvas.
Once the user enters a value to into the field for Angle or Length this value is fixed and only the other one can be modified on the canvas.
Of course the user can also define both values by typing in the values.