I'm going to agree with Bryan's comment on that site:

"Don’t see the point. You’ve got very little difference between carrying around that other upper and mag block and buffer/spring, etc vs. just carrying the other upper + another lower. And with that combo I can switch between 5.56 and .308 in however long it takes me to set down one and pick up the other.

With the exception of some sort of legal situation where you can technically only own 1 lower, this seems pointless"

For that price, you can own a 308 and an 5.56 AR (granted, not 2 high end ones)...