You could also use a 32 qt. kettle on two gas burners (which is what I do if ya got one) and boil 5 gallons of water and start adding your x2 ingredients. Before this, if you have steeping grains, you can steep in a smaller 8 qt. kettle with 2 gallons of water and add that when finished to the 5 gallons of boiling water. This would give you about 7 gallons and some head room for foaming. You'd have to add some make-up water in the primaries of about 1.75 gallons each after boil-off. Really, two gas burners under the one pot does work well as I've done it twice already with great results. You may have a different set-up but this is what I use to get the most of a full volume boil for 5 gallon batches. This is what I'd do to get as close to I can to a full vol. in a double batch. You'll definitely need a wort chiller. I got a SS one that takes near boiling water to 72 degrees in about 10 minutes for a 5 gallon batch.