On the machine that handles the SIP phones (Aastra 6757i/31i or softphone), there is no DAHDI installed. Just Asterisk 188.8.131.52 and the Asterisk addons. (We had started with trixbox but ended up just doing Asterisk and handling the config files ourselves)
The machine that handles the SIP phones has two network cards. One to the local network and the other is directly connected to the other Asterisk box. A dedicated connection between the two for the IAX connection.
The other box we call the bridge and it has the telephony cards (DAHDI is obviously installed on this machine). Its job is to handle bridging the incoming calls to either the old meridian system or over to the new system. It also accepts the outbound connections from either machine and gets them to the PSTN.