beamforming /steering vector for planar array

Here's to answer to my own question. Hope it might help others in their
projects.

Use a LMS approach for the fitness function. For example, yr desired
fitness, Fd is 0.01. The fitness solution of null1 and null2 is represented
by Fs1 and Fs2. 

Thus yr new fitness is produced by combining both fitness1 and fitness2:
(Fs1 - Fd)^2  +  (Fs2 - Fd)^2

It is basically that simple and it works perfect.