In this scenario, assuming Trueheart Duelist has no damage marked on it, you can have one Khenra Charioteer assign 2 damage to Trueheart Duelist, and 1 damage to the player or planeswalker it's attacking, and have the other Khenra Charioteer assign 3 damage to that player or planeswalker. This is because lethal damage will be assigned to Trueheart Duelist, the only blocking creature in this scenario (here, 2 because its toughness is 2 [C.R. 119.6]), so that you can choose to assign the leftover damage by each creature with trample to the player or planeswalker it's attacking (C.R. 702.19b).
EDIT (Apr. 16, 2018): Correctness edit.