Find a way to monitor pin-10. When active , there should be a 3.3khz squarewave there. You can connect a led with a resistor to it and it should light when the device is active. If the led turns on for both devices when they share a common power source , then they are working and your inability to read the ASCII output is a separate issue. Once you confirm that they are in fact both working, regardless of your ability to read the ASCII, then you can focus on fixing the problem. You are not going to fix a serial port conflict as long as you are chasing a red herring with the power . Find out if they are working using leds. Ignore the serial port.