if the antenna have the same azimut and the are set in the same row, the minimun distance is about 1,50 meters. one antenna is a bit in front of the second, they should not have the same azimut. for example, if both have 0° for azimut. you had to change the azimut for the behind one if this antenna is at the right side of the second one. Then the azimut will be 20° for example.
I hope it was helpful for you