For those who are not mashing on a system large enough to hit high gravities for a Barley Wine or Imerial Stout, this technique can provide a solution, if one has the time that is. This method will require some refinement as results may vary depending on equimpment, but I have made beers on a 5 gallon system of over 30 degrees plato! The idea here is to not only expand the universe of brews one can make on a small system, but also add a little twist (in my opinion). When I do the first mash, the first runnings are collected in the brew kettle, and those first runnings (the first gallon and a half or so) are put on the burner and boiled with a little bit of hops for the full time the rest of the sparge and through the second mash. This will raise the gravity, add color and a little carmel rich flavor to the beer.
Do a full mash both times, with a full sparge. The after collecting some first runnings, fill up the pot that is used for mash in water with wort, and just stop collecting when full. Dump the first grains, heat up the collected wort, when at temperature mash in the second set of grains. Sparge as usual into the kettle that is holding the boiled high gravity first runnings.
1) Add some rice hulls to the second mash. Sometimes the sparge will stick due to the higher gravity. Rice hulls will eliminate the problem.
2) There is no cutting corners, this method will basically take all day. When shooting for higher gravities, make the first mash bigger, and collect those higher gravity first runnings for the boil.
3) Keep an eye on that boil, as I have come close to syrup in the past.
4) I have used this technique in making a double bock (recipe will be posted soon) where I use 2 decoctions for each mash. Makes for a really nice dark color using 100% Munich malt.