The radill of two soap bubbles are r1 and r2. In isothermal conditions two meet together is vacum Then the radius of the resultant bubble is given by
When two soap bubbles combine, the volume is conserved. The relationship between the radii of the initial bubbles and the resultant bubble is given by $R^3 = r_1^3 + r_2^3$. Simplifying for radius in terms of the areas, we get $R^2 = r_1^2 + r_2^2$.