Staff with both "EARLY BIRD" and "NIGHT OWL"?

In doctor mode, you can freely select Perks of staff, and you can change the number of Perks that can be selected by editing xml. (Editing xml is a kind of cheat... but it's very easy and convenient...)
I wondered, how do staff members who have both "EARLY BIRD" and "NIGHT OWL" work?
My guess is that both Perks suffer from some useful buff effect on efficiency when working in a time zone that corresponds to Perk,
I think that there is no debuff in the time when it doesn't conform to Perk, it is just “no positive effect”.
So if you have both Perks, you can get the useful buff effect no matter which shift you work in,
I think they can work with high efficiency without being offset by debuffs that are out of time, is that true?

If this guess is correct I would edit the xml again... lol