Of the letter which she was writing she knew nothing whatever.
"Leonie II.'s independent action is not entirely confined to writing letters.

She observed (apparently) that when her primary self, Leonie I., discovered these letters she (Leonie I.) tore them up.

So Leonie II.
hit upon a plan of placing them in a photographic album into which Leonie I.could not look without falling into catalepsy (on account of an association of ideas with Dr.Gibert, whose portrait had been in the album).

In order to accomplish an act like this Leonie II.

has to wait for a moment when Leonie I.is distracted, or, as we say, absent-minded.
If she can catch her in this state Leonie II.