(Usual wall of text warning)
If anyone is suggesting Brown is a better player than Tatum they are pretty delusional I am afraid.
It is fair to want to trade Tatum as you will get more back, but, I don't think you will get a package that will make the current team better. If you are trading Tatum because you think Brown is *better* then I don't see it.
Pick an advanced stat, any advanced stat, and Tatum is a better player offensively (clearly) and defensively (marginally). He is also almost 18 months younger.
They run a similar usage rate and while Brown scores efficiently, that is all he really does. He turns the ball over more frequently, involves his team mates less and is weaker on the boards. From the (admittedly limited) Celtics games I have seen the team is much more likely to run offense through Tatum when both guys are on the floor whereas Brown has been more a transition guy. I assume teams know more than we do and it is not dumb luck they prefer to put the ball in Tatum's hands more often than not.
Now trading one of them certainly makes some sense, as to me, their team lacks a lot of direction and identity and I feel they could use a real lead guard. They are a slow-paced team that is just moderately efficient or below everywhere offensively. I think Tatum is stretched as the primary ball-handler and Brown and Smart are ill-suited to the role. Schroeder is fine in limited doses especially with the second unit but you don't want him to be they guy taking touches from Tatum and Brown.
The hard thing is finding a guy to trade for who a team would move and would make the Celtics better. Simmons and Lillard are the obvious big names which are floating around but would either Philly or Portland move one for Brown (they would for Tatum in a heartbeat). I think both would ask for a lot of picks in addition to Brown and Boston would probably not give them up.
Both would certainly help give Boston an identity and I would be intrigued defensively with a Simmons/Williams/Horford/Tatum/Smart line up though you would likely get the same offensive spacing issues Philly had. Lillard would boost the offense greatly and Smart could handle the team's best backcourt player each night reducing Lillard's main weakness. However, how much more do you give up when you are swapping a 25 for 31 year old?
Outside of those two who do you have; Sacramento? The Kings always seem up for something funky and in Fox, Haliburton and Mitchell they have three young PGs. Would they do a Brown for Fox trade? Would it help Sacramento? Is Brown really that much better than say Hield who they are already shopping? Would they do Brown for Fox and Hield? Maybe not? (Again they would for Tatum). Does Fox even make Boston better?
San Antonio? What about a Brown for Murray trade? Murray has quietly become a very good offensive player while dragging his outside shooting up to reasonable. Defensively he is a beast. He probably isn't the true lead guard I would be looking for though. No idea if San Antonio would be interested either though they were apparently kicking the tyres on Simmons.
Houston? Wall's contract is terrible but he is most likely still a cromulent lead guard (how can you know when he has not played!) and you could probably pry Christian Wood plus what else away from Houston? Most likely too risky and Wall having a PO at 47mill for next year most likely makes it a non-starter even if the cap maths could work.
I guess the fact you even have this discussion highlights the immediacy of results the public demands these days — Tatum is 23 and Brown is 25. Neither has even reached their prime yet and we talk of trading them away. Yes, the last two seasons have not gone as planned but they have won five playoff series and been to the ECF twice already. Even Jordan was 26 before he achieved either of those things!