Wireless transmission power for multiple receivers
When an isotropic antenna transmits power it does so in all directions. At some distance R all that power is passing thru the surface of an imaginary sphere of radius R. At distance 2R, the same amount of power is passing thru the surface of a sphere of twice the radius etc etc..

An isotropic receive antenna placed at some distance to the transmit antenna can only collect a small fraction of that power. It has, what is called, an effective area  (or antenna aperture) and this is measured in sq metres.

This means that two isotropic antennas (that don't overlap) can each collect their own power and the transmit antenna doesn't know - it believes (if it had the power to reason) that all the power transmitted is going to reach the far ends of the universe.

The same is basically true of any antenna - providing the effective apertures don't overlap they'll each collect the "theoretical" power transmitted by that sort of antenna.

All this falls down when you get to the near field because in the near field you can effect the transmit antenna by loading.

Think of it like a light-bulb - several people in the room can each see the light but if one person stands in front of another then somebody misses out.