Barley Wine and Parti-Gyle

That's what I do. I used to do 3 but time restraints make it easier to brew 2 batches for me. I usually just cap to the style and strength I want and all is fine. Just watch the water temp you add to the 2nd mash. Try to hit it right.

 

I did a barleywine/parti-gyle last year, just getting around to drinking the barleywine. adding grains for the 2nd beer is called "capping the mash" and it's okay to do, it needs a bit of a rest though. be prepared for a long day though. For the 2nd beer I made I pitched a belgian yeast and it's one of the best beers I've ever made

 

Fuller's brew all their main beers from Chiswick Bitter (3.5%) up to Golden Pride at 8.5% from the same mash via parti gyle.The details about how they do it must be available.

 

I am not sure what you mean by "remaining dry mash."

You can certainly do what you planned. I have done it before and the beers can be a bit thin if you are just sparging more from a planned 5 gallon batch even if it is a big beer.

As others said you can cap the mash with other malts. What I have found works pretty well is to perform a second mash with a few pounds on the stove while the main mash is going. Then cap the mash with that and proceed. Adds a few quality gravity points.

Good luck!