But the big deal is that both of those HRs were off of Hall of Fame reliever Bruce Sutter. The game was televised nationally, I think on NBC. It was, if I recall correctly, a warm sunny day at Wrigley Field (I saw this game on TV).
Sandberg was not a big star yet, although he did win the MVP award that year. It was only his third full season and he had not yet played in an All-Star game. His career average entering the season was .261 with just 15 HRs in 327 games (all statistics used here are from Baseball Reference).
He did enter this game with a .321 average but just 7 HRs in 66 games (a pace of only 17 for a full season).
But how hard was it to homer against Sutter? He gave up 9 in 1984 while facing 477 batters. He faced Sandberg 6 times that year and gave up those 2 HRs. So when not facing Sandberg, Sutter gave up 7 HRs in 471 PAs. The year before he gave up 8 in 384 PAs. When not facing Sandberg over those two years, his HR per batter faced rate was 1.76%. This sounds about average. The NL HR per batter faced rate over those two years was 1.81% (again, without IBBs removed).
He also issued 18 IBBs, so that HR rate might be higher than average. Sutter had a higher IBB rate than average, which was only about 1% (Sutter's was about 2%).
So yes, it was a big deal to hit those HRs. But at this point in his career, Sutter was about average in allowing HRs. In fact, not counting Sandberg, he allowed a higher HR% vs. righties (2.4%) than lefties (1.6%) over those two years.
Sandberg has said "It was one of those wild games, wind blowing out a little bit so it was going to be an offensive game from the get-go." See Wrigley 100 June 23: The Sandberg Game. There was, however, only one other HR in the game.
So what was Sandberg's WPA in that game? 1.063. It is the 35th highest since 1941, according to the Baseball Reference Play Index. Here are the top 40: