General expressions:
* Parler à côté de la plaque: This literally means "to speak beside the plate" and implies that the two people are not on the same wavelength.
* Parler d'un autre sujet: This translates to "to speak about another subject" and is straightforward, though it might not capture the feeling of being completely disconnected.
* Être sur deux ondes différentes: This means "to be on two different wavelengths" and is a more figurative way of describing the situation.
More specific expressions:
* Parler chacun de son côté: This translates to "each person speaking for themselves" and emphasizes the lack of interaction.
* Parler comme des sourds: This means "to speak like deaf people" and is a stronger expression that implies a complete lack of understanding.
* Avoir des conversations parallèles: This means "to have parallel conversations" and implies that the two people are talking at the same time but not really engaging with each other.
The best expression to use will depend on the context and the specific meaning you want to convey.