I think it would be different if the chap in the RHS held a valid instructor rating. Otherwise, he's not qualified to give instruction and therefore the LHS chap has to fly on the ratings in his licence. If he has no valid IR for the aircraft he's flying, he can't fly it in Class A in the UK. RHS would have to be P1 for the Class A bit - and for any flying where LHS was not "rated".
Once you've decided who was legally entitled to be P1, you have decided who can log it.
Flying in "simulated IMC" (eg under the hood) while holding a valid IR (or IMC in the UK) is different. You just need a lookout in the RHS.