I don't really care what people write in their log books and tend to be surprised by how much time people spend discussing whether you can log "pilot bloke sitting in the back making unhelpful remarks" or similar.
Isn't the simple rule that (leaving aside exemptions) when two PPLs fly in an aircraft which has a minimum crew of one pilot, only one of them can be P1 at any given moment in time. They can decide between themselves who is P1. In the case of a checkout on a complex type, however, if one of the two PPLs has not yet received differences training for the complexity, then he/she can't at that stage be P1.