I did a typical mid-day brew, drank a 6'er of guiness while out in the sun working on my tan. That brew went off w/o a hitch. About 7 pm or so my buddy shows up as I'm pitching yeast on to the first brew. We sampled a few higher gravity brews and began firing up the second batch.
Second brew, after 10 beers and many hours of sun was going good until.... I realized I mashed with the ENTIRE water volume for the entire batch, sparge and all. We essentially mashed with like 9 gallons of water. It began pouring down rain. We had so much freaking wort pre-boil we had to literally crank up the propane to max and continue drinking with our friend out of the rain. My friend cut himself somewhere along the way bleeding like crazy. He ate a whirfloc tablet. We got soaked.
We boiled like CRAZY for an hour.. then started the normal boil schedule, late into the night, after dark, after a LOT of alcohol. The next day I woke up and looked at the carboy. Half of the yeast ended up outside the carboy. There was sticky crap all over the side. I had a HUGE headache. However that beer fermented like crazy, and after about 4 weeks into the keg (we of course kegged it green), it was unbelievable. We could NEVER recreate that beer, rain water, maybe blood, and HUGE f ups, but it was so good.
So you're likely to get some fatigue by the second brew but my advice is to start early and drink lightly for the first brew, you'll still have a few hours of brewing left to go. Don't start throwing back the beverages until the boil of the second one starts!
My advice is just go for it! Something so satisfying about having TWO carboys bubbling away the next morning