Actually Dark Eldar and Craftworld Eldar are known to work together. Yes, they are enemies but they are still both Eldar and will fight together against a common enemy. The Harlequin often works as ambasadors to negotiate a temporary truce.



Some craftworlds are known for having very close ties with the Dark kin and actively work alongside of them (The il-Kaithe craftworld specifically)



It generally boils down to the individual craftworld and Kabal. They range from archenemies to BBF