61.195(f) says you may not give instruction for a certificate or rating until you have 5 hrs of PIC in make and model. So you can still do instrument proficiency checks, flight reviews, and give other instruction, it just can't count toward a certificate or rating. So in theory, you can have someone walk in with zero time, and you can instruct them in a twin, just the first 5 hours can't count toward their private pilot certificate. Not that you would do that, but the regulations don't prohibit it.