We can define groups of workers who must either work together or apart.
Example:
Anatoly and Barbara are to work simultaneously (the same shifts) on Mondays and Tuesdays (This applies to all shifts). Additionally, these two workers are to be kept separate on Saturdays and Sundays (between the hours 06:00 and 15:00) (They will not be assigned shifts, - even when those shifts overlap for just one hour, or even less - that would in any way result in them working simultaneously on either a Saturday or a Sunday).
As for Claudia and George, we mandated in the example above, that they are to be at work simultaneously on the exact date of 20. 7. 2016, exactly between the hours 06:00 and 15:00.