For high isolation it should be 0.5 lambda which is 35mm and then you can reduce it to desired distance and can get the results you want

Thank you So much Saeed Bhai.

Also you can enhance the isolation between both antennas without using exactly the half wavelength criteria. For example, you can use slots, SIW, stubs, or grounding techniques.

Hamza Sir Yes i have achieved isolation upto good extent however i am not familiar with stubs can u recommend an article for it?

DOI: 10.1109/LAWP.2014.2360353

The authors introduce the stub Ls between both antennas to enhance the isolation. This technique is usually used in cpw technology.

Good luck