Hmmm, I'm inclined to think his strategy is designed to make the best use of trellis space. But you can only pack 'em and stack 'em up to a point. Peas and beans are planted closely and grow densely; so mixing them up isn't likely to increase the yield. And yes, it will get messy. He will need to leave the pea vines wither in place to avoid damaging the beans.