Can I connect two separate power supplies using the RockBoard Power Ace Current Doubler and RockBoard Power Ace Voltage Doubler?

Yes, if both power supplies have isolated outputs and the same specifications. As long as both input sources have identical current or voltage, it makes no difference if you have two separate 9V DC power supplies (with 300 mA, for example) or two isolated outputs of a multi-power supply with the same values. The Current Doubler and Voltage Doubler were actually designed specifically for multi-power supplies.