Minimum separation between antennas in receiving
Low power radios are fine with the antennas up close. By close I mean within 2 wavelengths of each other. For instance at 300 MHz, the wavelength is 1m and a receive and transmit antenna spaced less than 2m isn't going to cause any problems. Both are at the verge of operating "near-field" but there's not much power so no real damage can be done. Bring them closer and again the worst that can happen is low signal power is received due to reflections and interactions but, you would have to try hard to make this happen.

Problems can arise with higher powers than just a few tens of milli watts. Close vicinity means one antenna will cause the other to be mismatched to its feeder and some power will be returned to the power amp (PA). At a few hundred milli watts or higher, this can cause damage to the PA.

But, think about a yagi antenna - it uses extra elements (that act like individual antennas) to form a tighter beam and benefit from "gain" i.e. more power is sent in one direction so, properly designed, it doesn't have to be a problem.


  It's known that there has to be a separation between antennas of same
  frequency to minimize interference.


Antennas operating at the same frequency shouldn't be used as two independant systems - this is asking for trouble and you will likely get deep cancellation as signals "phase" in and out.

Also, even operating at different frequencies it's not a good idea to have transmit antennas up close without careful consideration.

If you wanted to transmit 300 MHz and (say) 310 MHz you might as well use one common transmit antenna and add the signals together before the PA. The proviso is that the bandwidths don't overlap.