Thank you all for the replies.
The company does not have the approval of multi-pilot operations, so we fly it as a single pilot plane (VFR day). But there are instruments for flying it from the right seat as well.
The line training captain is signing the technical log and the company said to me, that the flight shall be logged as PICUS.
I contacted one of my flight school instructors about the ordeal as well. He said, that in the end, it does not matter, if it is PICUS or PIC. Either way I am getting PIC hours, that is the point that matters.
So right now I'm following my company's instructions and logging it as a PICUS flights.