User blog comment:B214/Question and Answers/@comment-50.11.177.49-20130502005354/@comment-33447191-20130502012415

They're both Satans however, Vali is the direct descendant of the original Lucifer while Sirzechs is not.

Like you said, there has been no interaction between the two of them, however, the both of them are aware of the existence of the other. During the negotiations between the Three Factions in Volume 4, they were both in the same room.