Connect the output of an Arduino to the input from another Arduino
As you are trying to communicate with two arduinos I will suggest you to use some sort of standard communication protocol such as Two wire interface or I2c. There is the Wire.h library you can use and it has also a reference on the main Arduino site. If you have different necessity and want to carry on with you're methods you need to specify in more depth the connections you have made. As for now I can guess that the output is being connected to ground or some sort of load and that might be the reason you are seeing a drop out of voltage. But it might be also the case that you're output pin is changing so fast that the multimeter is not capable of reading the high state correctly