I have done three batches in one day before. It was exhausting and I ran out of gas during the third (refilling station nearby) but I would suggest:
1. PREPARE THE DAY BEFORE:
-Weigh and crack grain the day before. Make sure each grist is easily identified (can be as easy as a small piece of paper inside the container you're holding your grain in with whatever brew it's going to be put in)
-weigh and label hops additions and make sue it's clear which goes with which brew. A tupperware container with a piece of masking tape is enough. Eg - one says APA 60, APA 30 ESB 45, etc
-write out your brewing schedule clearly and run through it to see if it makes sense. Include things like gravity readings where you would normally take them (1st runnings, pre-boil, post boil etc)
-make sure all necessary equipment is cleaned and ready and where necessary sanitised to the point where some no rinse is all else that's needed just prior to filling/chilling/whatever.
2. ON THE DAY:
-Start early. Heat your 1st brew's strike water at the same time you make your morning coffee. Don't get up at 1 pm and think 'yeah double brewday'.
-Organise which mash goes first. If one is 60 minutes and one is 90 minutes, do the 90 minute second - gives you a chance if anything goes tits up with the first boil.
-Do everything you can to avoid a stuck sparge (rice hulls, lessen stirring etc)
-Clean the tun and get the second mash on as on as soon as the first is on the boiler
-Tick off the stages you're at with the first while mashing and make sure you record times of each. I use an alarm but if I have 2 things going does the alarm mmean add [email protected]
or does it mean mash out? Notes help distinguish these. Pen and paper is a must.
-Drink beer very slowly and not until after first brew is on the boil.
3. Following the brewday, clean as much as you can be stuffed (and follow up the next day what you don't get done), relax and have a brew or 6.
MAKE SURE YOU HAVE ENOUGH GAS.