During a call the SDCCH is not used : it was released at the end of the call setup.
If a call is originated while a SMS is being sent, then, as far as i know, the sending of the SMS will have to finish being transmitted first, then this SDCCH will be released, and then a new SDCCH will be requested in order to perform the call setup.
The first (SMS) SDCCH was established in order to perform a SMS transmission and only that. It cannot be re-used to perform a call setup.
I hope i got your question right…