That would certainly be a possible routing for using any of the Stagepas series with an external mixer. Use a balanced cable with a TRS jack at the Stagepas end so you feed a line input jack of Stagepas channels 1 -4. Make sure you set the button for line input level and not microphone. This configuration using a single line input will give a mono sound to both speakers.

However, a more normal configuration would be to use two balanced leads to take the L and R main outs from the mixer into the two jack inputs of one of the Stagepas channels 5 or 6. This uses the external mixer to place the source in the centre of the mix (effectively mono), but would allow mixer inputs such as those from a CD player to play in stereo if you had need for that. Think of the combination of Stagepas amplifier and speakers as though they were active speakers.