Ricin beans complement each other's protein content. There are 9 amino acids which are necessary for a human bean but our body cannot produce them unlike the rest of the amino acids. So we have to get it through food. Meat, diary, egg are all contain all 9. There are some more sources, but generally plant based food do not have all of em. But rice has some, and beans has others, if you mix them you get all 9.
On the other hand, when they started to eat ricinbeans, they did not know about this.