People say 3watts / gallon but I think that this is very generic. It really depends on how much you will need to raise the temp in the tank. My tank (210 gal)is in the basement too where the temp can get down to 60 but, with all the pumps running on it the water temp rarley drops down too much. If I have no heat on my tank will stay at approx 66 degrees and since yours is a 180 assuming a few pumps are on your system, I would think you would be compareable to mine.
But to answer your question YES 2 heaters would be safe, but I have run my tank easily at 78 degrees with 1 350 wattWon pro heat Titanium heater
. My LFS has them for 42 bucks with the dial in temp plug
at 250 gallons...to raise your temp by 10 degrees you will need 230 watts
At 250 gallons to raise your temp by 15 degrees...you will need 350 watts
20 degrees you will need 465 watts.....
I think 2 350 watt heaters will be fine if you hold a stedy temp without heaters at around63-65 degrees...JMHO