The coupling between resonant circuits is a well known and well described.

Change of resonant frequencies due to coupling depends on type of coupling and coup[ling coefficient. The eigenfrequency formulas give the answer. The theory can be found in my paper "Analysis of Coupled Dielectric Resonators by Means of Eigenfrequency Method", European Microwave Conference, Cannes, 1994, which is available at ResearchGate, or many other papers.

Hello,

Few references are attached for the help.

Thanks,

Is there any connection between antenna 1 and 2

Udhayanan s Udhai There is no direct connection between antenna 1 and antenna, except they share a common ground plane, FR-4 substrate, and 0.25λ distance between them.

In my view, check your radiation pattern. It will coincide with the second antenna pattern. It influences the frequency shift because the current length increases slightly. if you want to avoid, increasing the directivity of the antenna.