neville s
turn both on until the house cool. turn off the bottom AC, the cold air up stair will come down

chad.roscoe
With any air conditioning, if you know that you will need it for a few days, the best way to do it is to set it at a comfortable level, 69, or 70 degrees F, and keep it at that level all the time, and keep it running. You can move it down a bit when you need it, say, make the LR at 65 when you are watching TV, but the main expense in air conditioning, besides simply running it, is having it turn off and on; it will be cheaper in the long run if you keep it at a consistent level and on. Room air conditioners are going to cost more than central air. When I lived in Philly, using my AC added 200/month to my electric, but Philly was expensive utilities. So that will depend on where you are. I don't know that two units will cool the whole house. Ceiling fans may add to its efficiency, or closing doors to keep the AC just where you need it. Good luck~ c-r

oldehippy99
The first and best "trick" to efficient air conditioning is to thoroughly vent your attic. Be sure your attic has continuous venting under the soffits and adequate venting at the peak of the roof. Put a thermometer up there on a warm day. If the temperature is more than 10 degrees hotter than outside, add at least two "wind turbines" or a big fan to get that hot air out of there. Your A/C won't work nearly as hard nor cost nearly as much to operate. Consider adding insulation to your attic. If you live where you have to heat in the winter, the combined summer and winter savings can pay back the cost in only 2 years. The second "trick" is to air your house out in the middle of the night by opening windows both up and down stairs. Then in the morning, close the windows to keep the cooler air in. Close drapes on the sunny side to keep the sun from overheating the house. If you have south facing windows that you don't absolutely have to see out of, cover them with aluminum foil, shiny side out. I then use an Exacto knife to cut fine lacy patterns in the foil to admit a little light and to make pretty patterns on the walls. If you won't be using the upstairs rooms during the day, close the doors and turn off the A/C. It will only take an hour or so to cool the rooms off before they are needed. Following the above should save you about 30% on your A/C power costs.