Duncan only has 4HR on the season after hitting 20+ the last two seasons. Duncan is drawing walks and getting on base, so he is seeing the ball well and just needs to get that power stroke back that he boasted in seasons past. The only concern I have with Duncan is that he has broken down a bit in his previous seasons and his numbers tailed off a bit down the stretch. That being said, I think he still manager to put it all together this season once he returns to the line up. 20HR is still a possibility, and he will be hitting in the heart of the line up, so he will be placed in a run producing position.
Butler is known as a youngster, who at 22, was already being labeled as a DH type, all stick and no field. After a hot opening week Butler has tailed off a lot and is currently hitting .262 with 1HR. He seems to have been made the sacrificial lamb for a struggling Royals offense, that has struggled to score runs consistently this season. But if you look at it like I do, this is actually the Royals showing confidence in Butler. They know he can hit better and they know a stint eating up some AAA pitching will be just what his swing and confidence needs to get him where he needs to be.
If you have the roster space stash one of them if you think they can help. I have Chris Duncan sitting on my bench in one of my more competitive leagues and can certainly use his pop once he returns. If you don't have the roster space, just keep an eye on their situations and when they are called back up, make a call with your own roster to see if/ where you can use them.