Sharing the 5V between them with USB cables connected to both? Could it be done? Is there some kind of protection as it has when you connect both USB and power supply?
If the USB connection goes to the same computer you shouldn't have a problem. Else disconnect the 5V connection between the two Arduinos if you connect the other USB line.
I2C is not a network, it's a bus. And a slave cannot "login" to that bus, it just listens on the bus for commands from the master and replies if it's instructed to do so.
But Leonardo can't operate as a brigde between the Shield and the EasyVR Commander app (used to upload, voice commands to the module). Neither in PC mode (there are jumpers in the shield to choose which mode to use), used to upload soundtables, as it was expected, since the Arduino is held in reset just to interface the module thru the USB serial using the default pins (12/13).
That's something completely different. It seems that the PC mode just connects the serial interface of the EasyVR module to the pins 0 and 1 of the Arduino. Because the Leonardo has a completely different way of communicating with the PC over USB, this doesn't work out of the box. Do you know a what baudrate theat EasyVR Commander is communicating with the module?
To help you further with this stuff, I need the schematics of the EasyVR shield. If I don't know how this stuff is interconnected I'm always guessing and that's more than boring. Provide a link to the schematics of that shield.