The volume of the batter stays the same; when it is poured into two smaller pans, the sum of the volume of batter in pan1 and the batter volume in pan2, does not necessarily equal the original volume; there may be unused batter, depending on the pan size chosen. We don’t know how much batter the OP’s recipe produced nor how thick the cake batter was when poured into the 9x13 pan.
But we do know the volume of that pan and the volume of two alternative sized round pans. The total capacity of the pans must be taken into account, not merely surface inches, since the original volume may be greater than the combined volume of two smaller pans. The variable is what is the volume of the original recipe. It is the unknown variable. It is not possible to split 14 cups of batter into two 6 cup pans. You cannot put 7 cups of batter into one 6 cup pan. There is excess batter that cannot be used because it doesn’t fit.
