PaulS:
You WILL have to disconnect the two Arduinos in order to upload code.
I also ran across some USB doc somewhere that said that when the host is idle the lines are tri-stated so the device can initiate communication. Or something like that. I guess it's wishful thinking to imagine that to mean that on the serial side of a USB to serial converter (FT232 or similar) the lines are tri-stated when the USB host is not talking?
I guess what I'm wondering is whether both of e.g. host computer and some_thing_on_arduino_serial both have to try to talk at the same time for bus contention to happen, or if it works ok as long as they don't.