I just came across this video: https://www.youtube.com/watch?v=fxC5w3f8FkY which says that I can duplicate a trace on the bottom layer and then at the end just via it back up to increase the current capability of that trace.
Say for example I want to carry approx 5A ish on a trace. A trace calculator gives me the following 
I would require 68mm trace width. Is it possible to just do 34mm on the top layer and 34mm on the bottom layer? I.e each trace would be carrying approx 2.5A each. How would that work if the target device pin is SMD? I would need to via it up to the single 34mm trace, wouldn't that be a bottleneck?
Answer
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.
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