Tower Controller described confused emergency event involving an air carrier due to frequency and position assignments; noting the procedure used during this instance was less than efficient.

Narrative

The Tower staffing was CD/FD; two Ground; two Local; Cab-Coordinator and CIC. A B757 was a inbound emergency with one engine out. The CIC made the decision to combine grounds and have the other ground work the emergency discrete frequency. Normally; with Cab Coordinator open; they would handle the emergency discrete frequency. Once the Ground Position was combined; I had to monitor two frequencies. It became impossible to listen to multiple aircraft and emergency vehicles calling on different frequencies. The traffic complexity was so great; that ground traffic had come to a halt because of communication overload. Recommendation; ideally; the Cab-Coordinator would have been the best person to handle the discrete frequency. In the absence of the CC; the CIC would have been next in line. The main issue is the way that 126.4 frequency is published. In the past; 126.4 was a non-published frequency. When in use; if a user called on the main frequency; they would be told to use the correct frequency; but they would not be stepping on anybody. I would strongly suggest we go back to the way it was done in the past and not publish 126.4 but put the frequency on the ATIS.