I'm not sure we all get your question.
Are you asking whether you can mix forces from Crimson Slaughter and Khorne Daemonkin within one Combined Arms Detachment?
* No you can not. Like the people before me said, alle units within one Detachment should be from one Faction. Since Crimson Slaughter and Khorne Daemonkin are different factions, you can't put them in one
CAD.
Or are you asking whether you can take a Combined Arms Detachment of Crimson Slaughter and a
CAD of Khorne Daemonkin within one army?
* Yes you can. Allies means something else in this edition and isn't always an Allied Detachment. You will still have to choose a Warlord and the Faction he is from, determines your Primary Faction. So if you would choose the Chaos Lord from your Crimson Slaughter Detachment to be your Warlord, you could also add an Allied Detachment of Khorne Daemonkin, but also any other type of Detachment (like a Formation) from that Faction.