If the Raspberry Pi is failing to boot with the Expander Pi connected that normally means there is a short circuit somewhere on the Expander Pi.
Can you check that there are no solder bridges between the pins on the 40 pin GPIO connector. If you have a multimeter try checking the resistance between the 5V pin and ground and the 3.3V pin and ground. If the resistance is close to 0 ohms that would indicate a short circuit.
If you can not find any problems with the soldering on the GPIO connector there could be a problem with the Expander Pi itself in which case I will send you a replacement.