Here is my two cents worth ... done on the fly ...

The multiple feedpoints in a multiple tuned antenna give an apparent
increase in the real component (resistance) by transformer action
(currents divide by the number of separate vertical feeds).  However,
that does not alter the basic radiation characteristic of THE SYSTEM AS
A WHOLE.  The radiation resistance of the whole system is still related
to overall physical dimensions.  Multiple tuning is just a "trick" to
get an apparent increase in radiation resistance, but it does not trick
the laws of physics!

Where gains could be had is with reduced current density at the earth
mat around each feedpoint.  For a given resistivity earth mat, the power
loss is I squared R.  As the I values are divided according to vertical
feeds, then the current densities and I squared R losses are all much
lower than having one feedpoint for the given antenna structure, taking
full current.

Reducing the ground loss component of an electrically small antenna is
an effective way of improving gain (as the radiation resistance is
swamped by loss resistance).

In an electrically large system (like commercial systems) running high
power, the multiple feeds, each with earth mats, disperse the ground
current much better than for a single feedpoint system.  I have read
somewhere that there have been ground systems that have literally "burnt
out" trying to work with applied high power.

In summary, for a typical amateur system, size limited, I do not see any
advantages in trying multiple tunes systems.  Neither it seems do the
commercial NDB users, where "Marconi T" LF antennas are standard
installations.  They do however invest in rather good ground radial
systems ....

Regards,

Bob ZL2CA