Do you agree with this?
How close to a corner is "too close"?
I wouldn't worry about what those sites say. The only way to know where a subwoofer should go is to try it. Do the subwoofer crawl to find the best location OR buy a USB calibrated mic and download REW for free and take in room frequency response measurements. Some times corner loaded is best some times 1/3 the wall's distance from the corner is best, sometimes the best spot could be on a side wall or rear wall or rear corner. I have dual subs (which I will never go back to a single sub again) my subs in my room sound best with 1 sub up front on the front wall between the front left speaker and the TV/center speaker and 1 sub behind the corner of my L shaped sectional couch so that sub is on the right side wall about 2/3 the way back.
Also you MLP (main listening position) is just as important as sub location. If you are sitting in a null then you need to move your seating. You should never sit at the half way point front to back in your room you should sit in either 1/5 or 1/3 increments so that also means you shouldn't be sitting up against a rear wall or you will be sitting in a bass mode/peak.
What's the main benefit of using 2 subs? My room is only 13' X 17'.