My expertise is weak in this area, but just guessing.... using twin
coax in the above configuration, if the shields were grounded, would
allow the feedling between the antenna coupler and the feedpoint to be
'balanced' and yet the shields would not radiate as they would with a
single coax run.

Perhaps others, here, will either expand on this, or correct my
misconception.

Using the center conductors of two pieces of coax, with shields bonded
together, does create a balanced transmission line. Its
characteristic impedance is twice that of the coax itself. The higher
impedance of this feedline will cause the voltages on the line to be
higher than on a single coax, and the currents lower (all else being
equal, of course) and thus reduce I^2*R losses. This can be
beneficial if this line is being used between an antenna and a
transmatch/tuner/coupler, where a relatively high SWR may be expected
on the transmission line.

The OP might want to consider a two-coax run of RG-6 cable, for a
total line impedance of 150 ohms. Satellite-dish RG-6 is available in
both black and white - the white coax might help reduce visibility if
it's cable-tied to the metal mast.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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