Increasing PCB trace current capability by duplicating on other layer
Two coplanar  68 mil tracks  will rise  11'C with 2.5A on each track.

But the via  will add resistance to the other track, and the via  temperature rise will be higher so to prevent thermal gradient delamination stress, they should solder fill the vias.

But putting mirror tracks on opposite layers will raise the temperature of both tracks more than on the same layer.

The length does not affect the temperature rise since the power per length is constant and the total power loss is small.  This is because the resistance per unit length is constant and Pd=I^2*R here 15 mohms/" @ 68 mil. 

Yet trace length does affect voltage drop and inductance.