3db is minimal? I don't think so. Playing the same frequencies is not practical. As I said running one is a better option because it gets more power. Using the two different sub enclosures would be beneficial if you bandpassed them (i.e. you have sub 1 play a certain range, sub 2 plays a different range). There is only one amp so that isn't possible. The best solution is to use the dual 4 ohm sub (so long as it can handle the amount of power). I will admit that you can run both subs at 2 ohms, but it is not the best way to do it.