Chief Investigator

I am currently engaged in a conversation with someone who is quite adamant that a study can have a co-Chief Investigator and they have quoted several articles where researchers have referred to themselves using this term. I believe that there is no such thing as a co-Chief investigator and that there is only one Chief Investigator per country, responsible for the conduct of that study in that country. I could understand the use of the term if there was more than one country involved in which case there would globally be more than one Chief Investigator and they could collectively be known as co-Chief Investigators. However, this is not the case I am referring to, as these 'co-Chief Investigators' are in the same country as the Chief Investigator.
I can find no reference to co-Chief Investigators in the regulations or the UK Policy framework. Has anyone else come across this? Is it just an honorary term for a senior co-Investigator or mentor? If this is genuinely a real role then how is responsibility split and how is it documented?
I am told the NIHR are favouring this term although I can find nothing on their web-site.
Please help me settle this debate one way or the other.