It is perfectly obvious, at DC and low frequencies, the resistances of
the two inner conductors are in parallel with each other. So with a
fixed voltage at one end there will be a greater power dissipated in a
load resistance at the receiving end.

Outer conductor resistance has a secondary effect.

By connecting two cables in parallel the resulting attenuation will be
smaller. But as Cecil implies, for exact calculations, cable
impedance, cable length, generator and load resistances, and SWR enter
the argument.

In general, it is a complete waste of time, trouble and cost to
connect coaxial cables in parallel merely to obtain a different Zo
impedance.

If a reduction in line loss is the objective, then the most economic
and effective procedure is to use thicker wires in the transmission
line and let line impedance look after itself.
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Reg. G4FGQ