About current distributions and skin effect
The flux around each wire with AC current will cancel in between the wires when almost touching. This coupling reduces the  loop area as well which reduces the overall inductance. It also raises the capacitance between the wires and lowers the characteristic impedance. e.g. twisted pairs.

Skin effect reduces the cross-sectional area and raises the resistance as well as be forced to conduct in the outer surface depending on the frequency. This is because the eddy currents cancel in the centre raising the resistance forcing the current flow on the outer skin.