You can set soft and hard constraints. Soft constraints can be broken if the system is unsatisfiable otherwise.

For instance, by default, this config sets a soft constraint that two people must be at the desk, and a hard constraint that no more than two people and at least one person must be at the desk. Likewise, there is a soft constraint that desk time must be contiguous, and a hard constraint that desk time must be at least so long to prevent continual splitting.