The lesson I see w/ Tatum and Brown is that they can certainly play together and the team can do quite well with them on the floor, but you just can't design a gameplan around those two guys. They don't play off one another, they don't have a two-man game, and neither of them is an average or better creator.
So you have to design lineups, and an overall offensive system, that's premised on something else -- e.g. guard-big pick and rolls / pick-and-pops, or whatever -- and just take advantage of the great luxury of having two high volume individual scorers playing at the wing spots.
In other words, I think you should kind of pretend that instead of Tatum and Brown you just have a couple of average starting-caliber NBA wings. Such as, let's say, Will Barton and Harrison Barnes.
Then, you build a lineup around that, and a basic gameplan, that would consistently generate good looks and work well with those two players at the wing spots. The example I would like would be having a potent lead pick and roll ballhandler (take your pick of any one of like 10-15 guys in the league) running pick and roll with a lob threat (e.g. Rob Williams).
Tatum and Brown can function quite well in that sort of setup, and because they are actually more more offensively potent than your Will Barton / Harrison Barnes type player, they can occasionally get hot, break out of the base formula, and carry the team on a run. But you need a foundation that can consistently generate good looks which does not revolve around either Tatum or Brown.
In this way, my view of Tatum and Brown is that they raise the ceiling of your team, but they don't really establish a floor. Which is why the current team is so inconsistent. Because we're relying on guys like Marcus Smart, Dennis Schroder, and 35+ year old Al Horford to establish the floor.
Of course, on top of that you have the fact that neither Tatum nor Brown has consistently been effective at closing games. Which means you would ideally have that lead pick and roll ballhandler I mentioned before be a guy who has a long track record of creating and hitting clutch shots.