comments:
Yes, run them together. I'd run a window A/C as the primary AC, full blast nonstop (because it's more efficient and costs less to run), and use the central A/C as a booster, setting its thermostat at a higher temp (85F, maybe?) so it rarely runs, except in the hottest days when the window unit can't keep up. For instance, this new 10,000 BTU Midea U-Shaped is my new favorite, due to its 15 CEER. About $450 with tax included. Running it at full blast the entire month will cost you 480/496 kWh of electricity (for 30/31 days). I'm paying 10 cents per kWh (after having shopped well for the most affordable provider). At this price, this unit will cost me $50 per month to run at maximum cooling. And you can connect 2 of them to the same 15A circuit (but nothing else, please) because it sips power. So, if you buy 2 window units for $900, you will have 20,000 BTU of cooling power (almost what you have now with your 2-ton), but will spend only about $100 in electricity, even in the hottest months (recalculate for whatever you pay for electricity there). But they have variable speed and modulate down with the ambient temperature, blowing quietly only the cooling you need, and costing even less to run. The electricity savings can easily pay for the unit in a year or two.