subtract waves with different scaling

Hi,
I would like to subtract two data curves from one another. This is my situation:
Two 1D waves contain the y- and x- values of one curve, these two waves have the same scaling of course.
Then again I have two 1D waves for the other curve, but these two waves have a different scaling compared to the waves of the first curve.
Now I tried to use the Interpolate XOP for the data of each curve to bring them to the same scaling in an XY-wave in order to subtract them. But the problem is that the data sweeps in x, let's say it goes from [-100,100] and then back [100,-100], naturally assigning two y-values to each x-value. The Interpolate Function just averages these two y-values and sets the scaling as [-100,100], understandibly. But then my back and forth sweep is just one single line...
Is there any different way, for example one that just creates two equally scaled 1D x-wave and then do the same for the two y-waves?

Thanks!!