Big beer, small beer

I was just listening to the beersmith podcast. They were talking about creating a high gravity beer. They brought up brewing 2 beers with one batch. One would just use the mash liquid as the big beer and the other would use the sparge run off as the smaller beer. I know that the mash liquid is highly concentrated and the sparge liquid is a much more diluted smaller beer. I am just wondering has anyone ever tried this? what was your method?

the process is called parti-gyle. I've used it a couple of times when I know that I'll have a lot of sugars left over in the mash. you have to plan for it a bit, but you can double your production out of one brewing day.

https://www.homebrewtalk.com/wiki/index.php/Parti-gyle