The engagement of both autopilots for a planned manual landing was a no no at my employer pre retirement. This is due to the aforementioned up trim applied at 400ft. Not so much of an issue on the -800 with a longer moment arm and more elevator authority, but it is definitely more of a handful on the shorter -500 upon autopilot disegagement below 400ft potentially destabilising the approach at a late stage. The only reason to do it was to keep an autopilot engaged for a go around., whereas on a single autopilot approach, pressing TOGA causes autopilot disengagement.