Parallel busbars: how to orient?
Four things to consider

Stray inductance.
If you are after minimising stray inductance than option #1 is prefered

heat dissipation.
If you are are dissipating as much heat as possible, the larger the exposed surface area the better. thus #2 is prefered

proximity effect.
If you are after maximising the amount of copper you are after, you do not want the effective conductive area to be reduced. Akin to skin effect with AC but the general cause: proximity effect due to the magnetic field will reduce the usable area. Thus #2 is prefered

Attractive force.
If you are wanting to minimise the attractive force (due to normal conduction or during a fault) and cannot mitigate it via restraining bolts then #2 is best