If I create a plot that has two lines (X, Y) and (X2, Y), would I be able to apply the same curve fit to both lines?

If you create a plot that has two lines (X, Y) and (X2, Y) where the Y data comes from the same data column, you would not be able to apply the same curve fit to both lines. The curve fit data is stored with the Y data column, not the X data column. To apply two separate fits, you would need to copy the Y data and paste it into a new column so that you could plot (X, Y) and (X2, Y2). You would then be able to apply the same fit to the two different Y variables.