Contrary to popular opinion, you don't make have to make one beer with the first runnings and another with the second. You can blend them to get what you want. Read all the parti-gyle-related posts here:
Shut up about Barclay Perkins
I do it every time I brew. You will need 2 burners, 2 pots, and a big mash tun--then you get 2 batches in the time it takes for one, plus the time it takes to chill your second gyle. As far as calculations, use the efficiency you would with a normal batch, with one batch sparge of the same volume as your first runnings. The gravity of the first runnings will be about 70% of double the average, the gravity of the second runnings will be about 30%.
In other words, assume an English pale recipe: 6 gallons, OG 1.050, 80% efficiency in your system with a single batch sparge. Now buy twice as much grain as it calls for and mash it. Adjust your make-up and sparge volumes to make them the same, say 7 gallons if you boil off 1 per hour. You will still get 80% efficiency. The first runnings go into one kettle and will end up at about OG 1.070. The second runnings go into your other kettle and will end up at about OG 1.030. You will adjust your hop additions based on these expected numbers, but you won't hit them exactly no matter how careful you are (I'm an engineer, not a mathematician). You may also add steeping grain, or sugar, or whatever, to either or both. Boil. Blend. Ferment. Enjoy. Partigyling is awesome.
As far as recipes go, you can't go wrong with English ones, obviously. I started doing it this way in order to make 10 gallon batches using my small pots and the IC that fits in my small pots, AND to make English ales. I usually blend porters 50/50, which gives the exact same results as boiling both gyles together in one huge pot, like normal people do. i like to do a 1.040 and 1.060 pale ale. I am doing the 12-12-12 wee heavy with a 1.040-ish bitter. Etc.