Since your view was created with a LEFT outer join, it means that every row in the view will have a component that comes from “t_first”, and thus, the ROWID from t_first is used as the rowid for the view. If you had a RIGHT join, then the rowid from t_second would be the one used in the view, so your understanding about that is somehow correct.
Now, regarding the error, the problem is that if you don’t use the whole primary key to join the tables, the resulting view could have more than one record for a given record in t_first, and thus, the rowid in the view, which in this case is being taken from t_first would be duplicated, and that is not allowed.
So, besides using the whole pk to join, yes, a solution could be to select the rowid from one or both of the tables to create the view, but you have to keep in mind that even though that column was originated from a rowid, in the view it could appear more than once, or in other words, it is no longer guaranteed to be unique.
Hope I was able to explain it in an understandable way.