Doubling is generally close enough, but keep in mind there will be a difference in volumes as boil-off and losses won't necessarily scale.
For example. My pre-boil for 5.5 gallon batches is 7 gallons, 1 gallon boil-off, .5 gallon lost to the kettle and trub. For 11 gallons my pre-boil is 5.5 gallons greater at 12.5, but the others remain the same.