If the utility power sources can provide the voltage and frequency that the transfer switch is rated for, YES, it is okay to use the transfer switch between two utility power sources.
The automatic transfer switches are equipped with a microprocessor controller designed to monitor and recognize acceptable power sources, regardless if it is a generator or a utility power source.
For example, if the transfer switch is rated for 480V, 3-phase, 60Hz and the both utility power sources can provide this, then the transfer switch will work properly and be able to switch between them.
Published for:ASCO Power Technologies USA