First, please take note that Cross-Save and Migration are two different things. What you're talking about is the Migration. Second, the Migration can only be done once to a Steam account. You cannot migrate multiple Blizzard Battle.net accounts to the same Steam account. If you already successfully applied a Migration to your Steam account, that account cannot be used a second time. To Migrate your other Blizzard Battle.net account, you must use a different Steam account. There is no way around this; once that Steam account was used for the other Blizzard account, you, yourself, locked that Steam account out of being used for another Migration.