Independent doubles are a better way to achieve that than a "cheater bar".
Independent doubles are two tanks with a regulator and SPG on each tank. The tanks are then usually held together with a set of travel bands designed to make the tanks easily removeable.
It does require two gas switches during the dive as you use 1/3rd from the first tank, then 2/3rds from the second tank and then finish the dive on the first tank using the 2nd 3rd from it. This leaves a 1/3rd reserve in each tank and also ensures that should either tank or regulator fail at any point in the dive, that you will have enough gas in the remaining tank to get you back where you need to be to surface, exit, etc.