No, two in-phase speakers provide 3db increase, not 6db.

If you could double the signal and get 4x the power you could make
gazillions! Of course, you'd be creating energy out of nothing, but
who cares about the laws of physics?

Two identical receiving antennas would provide twice the signal voltage
if their RF outputs were connected in series.

One way of doing this would be for coax feed from each antenna to be
connected to the primary of a 1:1 RF transformer, and the secondaries of
the two transformers were connected in series. So if each antenna
delivered 1V from a resistive source impedance (R) of 1 ohm into a
matched resistive load of 1 ohm, the two secondaries in series would
provide 2V. However, the output impedance of the two secondaries would
be twice that of each antenna, ie 2R. To preserve matching, the load
would also have to be 2R.

However, the snag is.....
The matched power from each antenna is 1V squared divided by 1 ohm
(=1W), but the matched power from the combined antennas is 2V squared
divided by 2 ohms (=2W) - which is an increase of 3dB (and not 6dB).

Of course, if the receiver input was not matched, and its impedance was
much higher than R or 2R, it might be possible to benefit from adding
the two antenna signals in this way. Has anybody tried this?

I don't think anyone would try this because every television input is
either 75 or 300 ohms or both. Hoping for an input of 1000 ohms would be
a vain hope.