Collecting it all in a larger container and measuring the SG before splitting it into two boil kettles would work, but you could figure it out using "points" if you didn't have or want to use another container. If after sparging and before the boil you find that you have one kettle with 3 gallons of wort @ 1.063 and the other kettle has 3 1/2 gallons @ 1.038, the first kettle has 3 X 63 = 189 points. The second kettle has 3.5 X 38 = 133 points. Combined, you have 322 points and 6.5 gallons, or 322 / 6.5 = 49.5, or a SG of just under 1.050 for the pre-boil gravity. Boiled down to 5 gallons, the SG should be about 1.064.