Convert the crop 1 yield to monetary terms (price x grain yield) and then adjusted to crop 2 yield equivalent by dividing the crop 1 returns in monetary term by the price of crop 2 followed by adding to the crop 2 yield.
As an economist, I would suggest for monetary benefits [(Crop 1 output: main product & by-product x Price + Crop 2 output: main product & by-product x Price ) - Cost of cultivation of two crops] and this will be more appropriate than estimating the crop yield equivalent in a cropping system if your intention is to compare two systems.