You could grow it as a Maha Chanok on yr ataulfo rootstock and graft sweet tart onto same tree later. This would be quite a combination.
Although possible to multi-graft onto one tree, it would be rather difficult to control/balance, as sweet tart grows much bigger than Maha in his San Diego climate.
You are saying that the sweet tart will grow into a larger tree than the Maha Chanok.
Consider my planting location: I have a slope to the south side of my house. The idea is to have 3 mango trees going down the slope, one by one, forming a sort of vertical hedge.
Given that I want to create this downslope row, would it be smarter to plant the sweet tart at the bottom, and maha chanok at the top, so that the maha chanok doesn't get shaded out by the larger sweet tart and they eventually form a somewhat even 'flat' canopy.
does this make any sense? I hope haha