Combine 9V and 1.5V power supplies to make a 7.5V supply
The 1.5V supply will have current trying to flow in reverse through it from the 9V supply. If it were a battery there may be some problems with some technologies but because it's a power supply and more than likely uses a diode bridge, I don't think you'll find one that works: -







simulate this circuit &ndash; Schematic created using CircuitLab