According to Wikipedia, they knew each other when Simonson was starting out in the comic industry. They also shared an apartment with others at one point.
From the wiki article: At one point Simonson lived in the same Queens apartment building as artists Allen Milgrom, Howard Chaykin and Bernie Wrightson. Simonson recalls, "We'd get together at 3 a.m. They'd come up and we'd have popcorn and sit around and talk about whatever a 26, 27 and 20-year-old guys talk about. Our art, TV, you name it. I pretty much knew at the time, 'These are the good ole days.'"[6]
https://en.wikipedia.org/wiki/Walt_Simonson