The idea that Fiordiligi and Dorabella were left stranded in the wrong characters in the first part of Act I (up to Scene V) as a result of a change of plan during the compositional process is what I describe as the Two Sisters Problem. A significant number of translators of Così seem to have been aware this difficulty and tried to do something about it, which suggests that the problem was to some extent ‘common knowledge’. Various remedies were tried out:

Mozart's own solution to the problem caused by the change of plan was only a partial one. He switched text between the two sisters part way through the recitative ‘Mi par che stamattina’. The autograph starts with the text as given in the libretto, and the result is that Fiordiligi correctly names her lover as Guglielmo, although arguably the two sisters remain for the moment in the ‘wrong’ characters. At the start of Scene III, however, Mozart switched the libretto text between the two sisters, so that they are now in the ‘right’ characters. The 1791 Prague and Dresden versions (C1 and D1) both compromise by switching only the text of the first two phrases of Scene III between the two sisters, leaving the following dialogue as in the original libretto. In view of Mozart's contact with Guardasoni in the summer of 1791, it cannot be ruled out that this was his suggestion. Of the major Singspiel versions, Mihule remains faithful to Da Ponte, but a radical solution was attempted by Bretzner in Weibertreue. He had the idea of reversing the text sung by the two sisters in the whole of the first part of the opera, including the duet ‘Ah guarda sorella’. Thus it is Julchen (i.e. Dorabella) who begins ‘Sieh Schwester’. But there was a large flaw in the execution of this idea. Bretzner forgot that if he merely switched text between the two sisters, the wrong names would be bandied about. The effect is chaotic and it is hard to believe that this series of obvious gaffes could have survived rehearsal, let alone performance. The trouble begins in the recitative ‘Mi par che stamattina’.