Layout PCB High Current
Yes, if you have 150mil on top and bottom with both carrying half of the current, then each would get ~6A (or you could consider that both are 300 mil). This would give you a ~10C rise, if that is acceptable then use that. (Remember that it is resistance and temperature rise that determine trace size)

Also Make you do a current/resistance drop on the vias that you use, as they will contribute to loss and heating. (Saturn PCB makes a great calculator for this) Make sure vias are paralleled for the current they carry.


Source: https://www.pcbcart.com/article/content/copper-trace-and-capacity-relationship.html