Thad Young has not been good in a while. He is about 17% from 3 the last 2 seasons. Guys like him and Otto Porter and Rudy Gay keep on coming back but these guys are not going to help anyone at this point.
To me, it isn't so much of why not both Tillman and Young, it is simply why Young? It would come down to Young over Queta for the last playoff roster spot. I would rather have Queta. I guess you could waive Mykhailiuk to free up a roster spot too, but I don't see that happening, at least not for Thad Young.
At this point, it is probably no one (so Queta) but I would be looking at Gallinari and Oladipo as possibilities. The issue with either is injury and where they are in terms of recovery from their last injury. Neither would be expected to play much.