and that's the point. resonant transmission lines (antenna elements included) are not simple impedances. a signal received in an antenna sharing a common feedpoint or common transmission line with another antenna (even though they are not magnetically coupled) WILL be re-radiated by the other antenna, just because they do share a common feedpoint, and the voltages and currents present in one will be present in the other. if they share a common feed, it's impossible to have something happen in one without happening in the other.
There is nothing difficult or magical about antennas. If you have a single feedpoint, your circuit has only a single connection to the 'aether' and it may be treated as a simple circuit impedance. It cannot interact with any other part of your circuit as you have no antennas. Simply V = IZ (and in general Z = R + jX and will be a function of frequency)
If you have two antennas they generally interact, although some don't - for example perpendicular dipoles. Mind you if you plonk something in the nearfield that breaks the symmetry then you will cause them to interact. So with two antenna terminals, in general - using an impedance description:
V1 = Z11 * I1 + Z12 * I2
V2 = Z21 * I1 + Z22 * I2
so a current I1 flowing in antenna 1 can cause a voltage V2 to be induced on antenna 2, and vice versa. The elements Z12 and Z21 represent the mutual couplings. They are zero if the antennas don't interact.
Any circuit coupling through the feed network can be accounted for using simple circuit theory, it doesn't matter whether they are Ls, Cs, Rs or transmission lines.
So once you include the mutual couplings you can analyse the whole multiple antenna system using simple circuit theory.
Went looking for a reference to give you, see
wikipedia article
scroll down to the bottom to
"Mutual impedance and interaction between antennas"
I have designed large arrays of antennas. If you need to accurately control the amplitude of the radiation from each antenna you need to include the effects of the element interactions. This is the basis of how the analysis is generally done.