I also have a 48 qt (rectangular cooler) mash tun and I brewed an american barleywine with the first runnings and a damn fine ESB with the second runnings. What I did was a 1:1 ratio for the mash using 34 lb of grain, mash temp of 149. My recipe was shooting for a grain bill similar to Stone's Old Guardian, using mostly MO. Specifically, 91% MO, 4.5% each of aromatic and dark english crystal (135-165L).
The mash was super thick, and my sparge stuck, but I worked through it by draining what I could then removing most of the grain and adding more water back, thinning it out a bit. I don't have my notes with me, but I think the BW was mostly first runnings, maybe a bit of second runnings to get pre-boil volume. Three or so days into fermentation I boiled up some 3rd runngins that I had frozen, added some more hops and a bunch of sugar and added it to the carboy to help drive the FG down. I ended up at 1.025 FG at 12.5% ABV which might have been a bit too much, as bottle conditioning didn't really work for this one. It was still a great big thick malty beast, and if it had carbonated would likely have been one of my best.
My advice is to focus on hitting your numbers for the first beer (BW) and just let the second beer be what it wants to be. If you end up with a [calculated] post-boil in the 1.050's maybe plan on a hopping rate more for a pale than an IPA. My experience with partigyle has been that if you try to fit a single mash to two recipes, both beers will suffer. For example, even a 1.060+ beer mashed at 149 and then pitched with Cal Ale yeast will likely go below 1.010 with good pitching rates/health, which is pretty darn dry for most beers.
Good luck, partigyle is a great way to drag out the fun of brew day, and at the end you get two different beers!