Multiple independent antennas on same PCB
I can tell that this can work like a charm: I once designed a (professional) PCB with ZigBee and BLE (both 2.4GHz) on the same board with chip antennas spaced only 3cm apart from each other and 90deg orientation.

I do not say that there is no influence from one to the other, I just say that I did not notice any performance degradation when running both simultaneously.

It might have to do with the facts that ZigBee is on a static channel and BLE is pseudo-hopping on multiple channels and both have different modulation schemes.