There has to be a custom deal in place, but it could be similar in spirit to a hot standby MS SQL Server instance where you only pay for one license but have a

There has to be a custom deal in place, but it could be similar in spirit to a hot standby MS SQL Server instance where you only pay for one license but have a mirror of the machine ready to go if the first one goes down (my knowledge of licensing was only tangential to my dev work and could be several years out of date now).