For most of this season, he has played as the SF or a wing with two bigs. At times, he is on the court with only one big. I feel it is very clear that SF is his natural position, and he mostly plays that position.
When he is on the court with Brown, it is not real clear and probably doesn't matter which is the SG or which is the SF. They both essentially play as wings but Brown is more likely to cover the smaller, quicker (SG) wing on the other team with Tatum taking the more SF wing.
Even if Tatum is on the court with only 1 big, he still really plays as a SF or wing.