serial with both usb and tx/rx?

I'm working on a project in which I'd like to communicate between Processing using a serial port on a Mac and 2 different Arduino NG boards. I'd intended to serially chain the communication by connecting from my Mac to the first Arduino board via USB and then having the first Arduino board relay messages to the second Arduino board via a TX/RX pin connection. I'd hoped that when the first Arduino board resends a serial signal that it would be simultaneously broadcast across both the USB serial connection back to Processing and the TX/RX serial connection to the second board. As far as I can tell, though, the first board seems to not broadcast anything when the USB and TX/RX connections are both connected.

So, my basic questions are: (1) Am I right in concluding that this sort of multiple serial broadcast is not possible? (2) If it's not possible, can anyone recommend another means for sending the data from a computer to two different Arduino boards? I ultimately would like to just send a single byte across the network.

Thanks in advance! Casey

If you only need to receive bytes with the Arduinos you could tie together their ground and RXD (digital0) pins. Only the first Arduino will be able to send data back to the host computer, but they will both receive it.

If the 2nd Arduino needs to send data back then it gets trickier. You can't tie together the TXD pins since they are both outputs and would fight. You could take another pin on the 1st Arduino, tie it over to the TXD of the 2nd arduino and use the software serial implementation to listen on that pin on the first arduino, then forward the bytes out the hardware serial port.

Thanks, jims -- this is really helpful. I'm not sure why, but I hadn't considered just using the RXD pins on both boards. I don't really need the boards to send data back to Processing (all of the events are driven by Processing), so I can't believe I didn't think of it sooner. I guess I'm just so accustomed to using USB for serial communication and power supply that it hadn't crossed my mind...

Thanks again, Casey