In the mid-90's I worked for a company that tried conductive ink to create two-sided routings on single-sided CEM-1 circuit boards (cheap). They were trying to save money. Guess what? They didn't. It turns out that recalls and scrap are expensive.
That wouldn't work - the board still has two sides locally, it just has one side globally. So there could still be current leakage between the two (local) sides.