Yes, essentially that is how it's done. You would designate an aux send for left and another aux send for right, then dial in the appropriate send amount for each input to achieve the desired mix and placement in the stereo field.
On a Mackie mixer, the 12 o'clock detent on the aux sends represents a unity level. There may also be an aux send master pot for some sends depending on the exact board. The original 1604 didn't have any master send controls, just the 4 pots on each input that could be selected to go to 7 aux outputs (but not all at the same time).