I have two wave accounts currently (when I want to use one, I have to log out of the other one).
Is there a way I can import the data from the one and add it to the other one?
No, there's no way to merge accounts in that way, currently.
You could, however, add the primary email of one account as a collaborator to the second account. This would allow you to do most things on Wave from the same account. Otherwise, you'd have to re-create your second business under your first account manually.