Can you split track width among layers to equal one big trace?
At the very least, you need to do some derating as the two traces are not going to share current exactly 50-50.  When we parallel connector pins, we add 25% to the calculated number of pins and round up.  I think we do the same thing with wires.  So as a SWAG I would add 25% to  your calculated trace widths.

I don't think we've ever done that.  We would either increase the trace width appropriately, or go with heavier copper, which would be 2 oz Cu in your case.