After some thinking, I think the connector between both modules should have more than RX,TX,5V&GND:
at least two more pins connected to digital pins of the ATMEGA8.
Here is why:
I am using a wireless zigbee module connected to the Arduino Board. I have two copies up and running. One receives and the other sends the data. If I reset the receiver Board, it will still receive wireless data while it is loading and will therefore stall in bootloader mode! So what I thought is to use a pin of the ATMEGA8 to gate the wireless input data. When the microcontroller boots, it blocks input data unless it runs its code and sets pin 3 (for example) high to activate the bridge to the wireless input.
The connector could be similar to an ICSP connector (it should be polarized). I prefer connectors to "shields" because they faciliate the custom setup of the boards in small spaces (the can be set at various angles and distances).
Another advantage of adding pins to the connector would be a serial multiplexer. The ATMEGA8 could select between 2 different target devices. Here is the idea: