MAAZ
what about an europen airline that has got this SOPs.....
I agree with rubberdogpoo (!!!!), the word "acting" or "act" is probably of significance.
FWIW our EASA'land airline words the relevant section of it's Ops manual in this manner: "A Captain or other suitably qualified pilot may temporarily act as PIC in flight during periods when the designated PIC is taking rest.
He/she will remain under the overall authority of the designated PIC." (my emphasis).
So certainly in our case there's a single, designated PIC who has overall authority for the entire flight (and then elsewhere in the ops manual it is made clear they are the only pilot who can log command/PIC hours for the sector).