AutoCad :: A Line That Uses Common X / Y Coordinates

I have 2 different lines defined with x and y coordinates.

I want a third line that uses the common x coordinate and a y coordinate that is the sum of the two coordinates.

If this is possible, can i define which mathematical function i use so that i can add/subtract or even use a formula to define?

Further Notes:

The example above is very simple. I have 2 complex curves that go into positive and negative values of y (speaking from a mathematical perspective). I need an easy way to intersect the complex curves at a specified interval and add them at each of those intervals.

For example i may a sine function going in between pi and -pi over a distance equal to 2. I then need to split the curves in intervals of 0.1 and add them together.