There are several techniques to increase the gain or bandwidth of an antenna. Is there any guidelines or thumb rules to obtain 'Dual Polarization' or is there any technique(s) which can give confirmed 'Dual Polarization'?
I can suggest the following antenna elements with broadband perfomance: cross-dipoles, cavity backed cross-slots, capacitive patches with differential feeding. Once you have an antenna with linear polarization you need to use mirror plane to determinate the second feeding with a polarization orthogonal to the first one. Design symmetry ensures low cross-pol.