how transmission carry both power frequency(50Hz) and communication frequency (MHz) at a time
A transmission line, as a linear device, can carry many frequencies (like your cable television) without them interfering. You can then tap in and selectively filter for the signals you want. No reason they can't go down to 60 Hz, or even DC.
A MHz signal source/receiver can be capacitively coupled to a power line and see very little of the 60 Hz power.