I think if you have 2 separate public IP addresses, you can quite easily specify to have your exchage server as the primary MX on one IP and the SME as the secondary MX (on the second IP) at the DNS level.
What I'm unsure of is how SME's delegate mail server system works.
If it keeps automatically trying (and retries on failure) to send to the defined exchange server then it will work, I think.
If it only ever tries once to pass the mail onto the delegate mail server and gives up if the delegate mail server is unavailable / unreachable for any reason, then you will probably need to customise the mail server behaviour of SME (sorry, that is beyond me).
Kelvin