Well, maybe some of you have thought of this, and done it.
I wonder why, though, it's not a more common idea to have a heavy duty rack up front and full size panniers. The distance from pedal to axle is greater up front than in back. The weight of the bike is mostly on the rear wheel to begin with, so it's not as if the front wheel risks being overloaded.
One wouldn't want a rack that had the weight protruding past the front wheel, of course, because that would make steering more awkward. But, as I mentioned, there's a lot of room between the pedals and the front axle. One could get large panniers in front that didn't extend past the front wheel.
One might wonder about the strength of the forks, as compared to the rear triangle. But touring bikes, at least, have heavy duty forks, and one could always make them heavier duty still if that was thought necessary.
For what it's worth, I sometimes put shopping bag panniers up front and full size panniers in back to do errands. That works out well, although owing to the limitations of my front rack I don't load up so much as in the rear.
But why couldn't there be 50lb+ racks for the front of a bike, and panniers on it as large as full size rear panniers? Why doesn't anyone market such front racks for precisely such a set up?
One fear rack makers might have is liability. Many bikes don't have the forks for heavy front loads. Perhaps manufacturers are worried about their exposure if a heavy duty rack is installed on an innappropriate bike, and the forks crack. Just thinking out loud...