Ok - I guess the question is, is on/off per 100 the most reliable indicator of the issue, which is (apparently) a lack of good decision-making on offense? It seems to me we would want to slice the data involving close games, in the final 5 minutes, say, and the net result of a given play with those two guys in the same lineup on the floor.
In other words, following your logic, we would expect to see something around the fact that Brown being in the game in the above scenario ties in with going to cause Tatum to go cold, or make the bad passes, or vice versa. I don't think we've looked at enough data properly to presume that.
At one point last year I went through the games when Brown sat vs. Brown played. I looked at multiple season, but below I put the first part of the 21-22 season when the team as a whole was getting used to Udoka, but the dramatic difference in Tatum's shooting is pretty amazing. Tatum scored more on a better shooting efficiency in the games Brown didn't play at all. His other stats were in the same general range. It was easier to do it that way since Brown didn't play at all. I don't know if it holds in games in which both play when Tatum is on the floor and Brown isn't.
21-22 season
Games 4-9 (Brown played) - Tatum Stats
22.5 p, 7.8 r, 3.5 a, 1.0 s, 0.7 b, 2.0 t - 36.1% FG (27.5% - 3)
Games 10-17 (no Brown) - Tatum Stats
28.6 p, 9.1 r, 3.6 a, 1.0 s, 1.0 b, 2.9 t - 45% FG (41.6% - 3)
Games 18-22 (Brown) - Tatum Stats
20.6 p, 9.8 r, 4.0 a, 1.0 s, 0.2 b, 2.8 t - 35.1% FG (21.1% - 3)
Games 23-27 (no Brown) - Tatum Stats
31.0 p, 8.2 r, 3.4 a, 0.6 s, 0.8 b, 3.4 t - 48.6% FG (36.8% - 3)
Games 28-33 (Brown) - Tatum Stats
25.7 p, 8.2 r, 4.5 a, 1.7 s, 0.8 b, 3.2 r - 43% FG (33.3% - 3)
Now obviously that is less than a 1/2 season sample size, but because of when Brown missed time (in chunks), it is also easy to illustrate and doesn't just capture a hot or cold stretch for Tatum. When Brown played, Tatum shot poorly, when Brown didn't play, Tatum shot very well.
Brown also missed the 3rd game (which is why I started at game 4) and Tatum missed games 34-47 (why I stopped at game 33), but during the first 33 games that season, Boston was 7-7 without Brown and 9-10 with Brown (which is also a pretty consistent pattern year after year).