Not sure how al works. Please see attached screenshots. I tried al, it asks me to select one peak from each spectrum. Not sure what I should do for this? 

As you can see the two 2D planes from two different 3D NCACX are very similar, but not exactly the same. Their W2 15N and W1 13C(both indirect dimensions) cover slightly different range, one has W2[90.19, 146], W1[42.2, 71.92], the other has W2[91.9, 146.8], W1[43.76,74.75], so when I try overlay, nothing happens. What I wish to do is have them overlay on each other, its easier to use the assignments of one 3D to do the assignment of the other. Right now, if I hover cursor in the blue spectrum, I have a cross hair showing where my cursor is, but in the red spectrum, it only shows a vertical line.

Thanks!

Bo