Edit: correction, it appears that soybeans are the exception. But when one thinks of having a serving of "beans and rice", soybeans are unlikely to be the bean served.
But the truth is that you don't need to eat them in the same meal, and most people don't have a shortage of grain in their diets if they aren't going out of their way to make it so. Porridge in the morning and lentils at lunch and you've done it.
*I don't cite the source since I am unsure if there are caveats or if the claim is controversial.
Everyone always mentions beans and rice, but Potatoes by themselves are a complete protein. Some of the essential amino acids aren't found in high quantities in potatoes, so you have to eat a lot of them if you go that route. They are missing some vitamins and minerals as well, so you have to add sweet potatoes if you don't want scurvy.