Basic concept is you make a really big beer using alot of grain and a single draining of the MLT.
Then with your second and perhaps third drainings you boil it up as a weaker beer. The later beers can be beefed up with some specialty grains in the mash or some extract in the kettle. This way you get a second beer from the waste of your big beer with little costs.
The issues are that is can be hard to predict the gravitys for the later worts and the recipe is also difficult to pin down.
I'm not sure if it's possible to hit really high gravity numbers on the first batch and still have enough sugars left for the second. My brew day was far from perfect since I found out later I was consistently 10-12 degrees over my temperatures so it's not the best baseline, but at least it gives you an idea.
In total, I used 23 (18.5 on my first, and an additional 4.5 on my second) lbs of grain and got an OG of 1.082 on my first batch and 1.046 on my second. Both seem quite promising at this stage and I think they'll turn out well.