A good reference for the list of definitions and properties of double series is http://www.encyclopediaofmath.org/index.php/Double_series
The equality you are asking about is not true. Say, if a_1 = 1 and all the remaining a_k and b_l are zeros, then the left hand side is infinity (1 + 1 + 1 + ...) but the right hand side is 1. Moreover, the only case when the left hand side is a convergent double series is when all its terms are zeros.