I only have a 10 gallon brew kettle and make double 5 gallon batches regularly. I use a grain bill as if I was making 10 gallons, but only use enough strike and sparge water to yield about 9 to 9 1/2 gallons pre-boil. After the boil, I usually have around 8 gallons of wort. I split the wort between two 7 gallon conical fermenters and top off with water so each has 5 1/2 gallons. Essentially, I am making a higher gravity wort and then diluting it before fermentation.
It is easy to do and yields twice the beer. In addition, I experiment by adding different hops into each fermenter before pitching yeast and/or dry hopping with different hops and by using different yeasts. I currently have two IPA's fermenting. Both were hopped with Pacific Gem at 60 min., and Centennial at 5 mi. and flame out. When I split the wort into the fermenters, I added an ounce of Centennial into one and an ounce of Eureka into the other. I will dry hop each using Centennial and Eureka respectively. I also used Imperial Pub Yeast in one batch and Imperial Flagship in the other. While the beers will be similar, there will definitely be subtle differences.