What sort of setup do you have? I am really hoping to do 2 5 gallon batches in 8 hrs (excluding cleanup)
No problem, just Parti-gyle it! One mash, two beers. This limits the styles the two beers can be somewhat, but less than you might think. I have 5 two-batch days under my belt and they just keep getting better. The first 2-batcher was two separate mashes and was a long day. Ever since I've been doing Parti-gyle and it's easier and more fun. The last one was under 6 hours from heating strike water to both beers in carboys. But I have two brewpots which helps quite a bit.
With your equip, you could easily get a moderate (6-8% ABV) and a lower (3-5% ABV) beer out of a single mash in the 10-gal MLT. You'll need at least one bucket, two might be nice. And if you don't already, it's best to break up your batch sparge into 2 steps so that along with the 1st runnings you have 3 different strength runnings to use to blend your two beers to their appropriate pre-boil gravity. Put the first runnings in the bucket, the second in your 5-gal MLT (or 2nd bucket), and then once you dump the 3rd batch of sparge water in the MLT you can transfer the 1st runnings along with however much of the 2nd (and/or 3rd) runnings in as you want. Then start that one boiling, and in the meantime use the bucket and/or 5-gal MLT to blend out your second beer.
Depending on how much overlap in grain bill, you can do anything from just a big and small version of the same grist, or have the first (bigger) beer be all base malt and then steep specialty grains for the second beer. Here's the styles I've done just to get your imagination going, other combos are certainly possible:
Belgian Dubbel / Dark Strong (used some extract to bump these up a bit due to lack of MLT space. It worked out ok, but two high grav beers are more complicated)
Barleywine / English Pale
Belgian Strong / Pale Ale
Imperial Pale / Dark Mild --> my latest and favorite so far. 100% MO mash with just first/second runnings for the pale, and steeped all the specialty grains in the brewpot for the mild. Both beers were excellent, IMHO.
Here's just one of I'm sure many threads on some of the math involved in Parti-gyle:
https://www.homebrewtalk.com/f36/parti-gyle-please-check-my-math-277958/
Have fun! As it happens, I'm going to try for 3 batches this Saturday. I have 2 brewpots and going to borrow another burner, but I think I can do it!