"Foli a deux" is
The person with delusion influences the other to develop same delusion.
2 persons sharing same delusion  - "Foli a deux".
3 persons sharing same delusion  - "Foli a trios".
Sharing a delusion to group of people : Pseudo community.
Delusion of doubles
Substance abuse disorder
Sharing delusion with one more person
Dissociation disorder