Let's say I need a 20mm wide trace in 1oz copper to handle some high current. Could I just have a 10mm trace on one side and another on the other side, connected by a plated hole to handle the same amount of current?
Sadly, due to costs, I would like to avoid going with thicker copper on the board.
What matters is your total copper cross section and the trace length. Both of these numbers contribute to the total resistance of the trace (if that's important) and the resistance per unit length which contributes to temperature rise (if that's important). If it's a DC current, there are no issues that I can see. For higher frequencies, you might be generating magnetic noise if your return trace is farther away. (Magnetic noise is proportional to current x current path loop area)
Here is a website that can help you calculate the total amount of copper based on the target temperature rise: http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/
Edit: Another way to go on the cheap side is investigated by Dave Jones and Mike:
EEVblog #317 - PCB Tinning Myth Busting (14 minutes youtube video)
Does putting solder on high current PCB tracks help? (4 minutes youtube video)
Basically, leave your trace open on the soldermask, then go over it with a LOT of solder to beef up the trace cross-section. Another possibility is to just use a big fat wire to jumper it across your PCB.
Well, sort of. The two 10mm tracks will not handle quite as much current as a single 20 mm track, since the substrate will be heated on both sides and will get somewhat hotter. The big limiter will be the plated-thru hole(s). The total amount of copper in the hole barrels will not be anything like what is in the traces. If you're going to do this, use multiple holes (big ones) and solder some wires in the holes to beef up their current capacity.
