Not even in the frames of in-game logic? For example, if one realm swears vassalage to the other realm and the liege-realm wants the vassal-realm to put their banner as a term for the aforementioned vassalage.
That would be in-game illogic.
A banner is intended to be unique and identify that holder. Never would you pick the same banner, especially not for a vassal. People could think the servant is the master!
