Tipping bell services is alot like tipping in a restaurant, that is, everyone has their own way of doing it.
At WDW hotels, there is no way to know if the same person will be bringing your bags to the room or not. I've had the person actually point tidbit that out to me, attempting to get the tip right there.
If there is a gap between drop off and room delivery, I usually split the tip. That way, both handlers get a part of it, and nobody gets shafted. On the occassion where the same person takes the bag and stands nearby while you check in, and will be taking it to the room with you, I tip on the room delivery end only. (Note - This usually is not the case at WDW hotels, and I split it almost every time).