Thanks DR - you kicked me into finding the EASA recommendation.
In fact reading it, it is a bit wider than "2 crew of Flt Deck at all times":
The Agency recommends operators to re-assess the safety and security risks associated with flight crew members leaving the flight crew compartment due to operational or physiological needs during non-critical phases of flight.
Based on this assessment, operators are recommended to implement procedures requiring at least two persons authorised in accordance with CAT.GEN.MPA.135 to be in the flight crew compartment at all times, or other equivalent mitigating measures to address risks identified by the operator’s revised assessment.
Any additional risks stemming from the introduction of such procedures or measures should be assessed and mitigated.
The last sentence is a bit of a nightmare for airlines and regulators to assess... as some have pointed out, there are clear added risks to be evaluated by implementing the policy.
Which is "safer" comes down to someone paid more than me to assess thankfully
