Will a window ac unit keep my central air unit from working as hard?

Yes, adding a window unit will take some of the load off your central unit. However, it may not be cost effective. Check to make sure your vent is blowing about 14 degrees cooler than the room temperature at the ceiling vent. Make certain your suction filter inside the house is new and clean (not restricting air flow). Make sure the coils on your outside unit are not blocked. Make sure you see water dripping from the drip pan inside the house (its usually piped to the outside, normally a 3/4 inch PVC pipe. You're biggest problem may be insulation. The power company can usually provide a checklist of all the areas to check to improve insulation. You may have to seal the doors, caulk the windows, put heavy curtains to block out the sun. Limit the traffic through the door, etc. to keep cold air in. Keep in mine, the thermostat is what is controlling the Unit, so it should be in a central room, not in sunlight, and not obstructed. And make sure the thermostat is in Automatic and not manual.

A properly designed and sized a.c. unit will run continuously when it is hot outside. This is the most efficient configuration. Further, this has the most efficient humidity removal which makes the insides a whole lot more comfortable. Adding more capacity will increase cost (consume more energy) more than anything else and reduce the humidity removal.