Scoring when two triple word squares are covered in one play
If you cover multiple bonus squares, all of their effects are applied -- multiple letter modifiers first, and the full word score added up, then all multiple word modifiers are applied.

That means that if you cover two triple word squares, your letter count (including the one double letter you cover along the way, assuming it wasn't already covered) is multiplied by three, and by three again, so total of nine times the letter count before adding the Bonus for a full tray play.

Your 293 seems like a correct count.