What you are proposing to do is possible. Have one span go to the PSTN and the second go to the Definity. Then if an incomming call is not for a extension local to the Asterisk server, then send the call out the Definity Tie Trunk. I can;t be more specific because I don’t know Definity, I don’t know Elastix and I don’t know your system.
However, there is an option to re-use the phones and get rid of the Definity. There is company by the name of Citel that offers a box that will convert between several proprietary digital sets and SIP. I have not used the Definity emulation, but do use the Meridian mode and it works fairly well. Not perfect and not identical to the old way of doing things but it works and I did not have to buy 300 SIP enpoints and upgrade the cabling in our buildings. citel.com/Products/Portico.asp