I assume you have Basic? Need to hold Shift and select more than one in the tree on the left. The work-around for multiple layers/years in Basic. I think anyway, haven't used Basic in years and years 

That doesn't really work to do more than one crop at a time though since it's just averaging values. 


If you have Advanced, there's multi-year analysis that will give you a new single layer in the tree of normalized (multi crops) or averaged values.