Are two people required in cockpit at all times on large commercial aircraft?
EDIT: please see at the bottom of the answer the update from EASA

Are two people required in cockpit at all times on large commercial aircraft?

No.
From EASA, pag. 101-102: (emphasis mine)

CAT.OP.MPA.210 Crew members at stations
Flight crew members

During take-off and landing each flight crew member required to be on duty in the flight crew compartment shall be at the assigned station.

During all other phases of flight each flight crew member required to be on duty in the flight crew compartment shall remain at the assigned station, unless absence is necessary for the performance of duties in connection with the operation or for physiological needs, provided at least one suitably qualified pilot remains at the controls of the aircraft at all times.

During all phases of flight each flight crew member required to be on duty in the flight crew compartment shall remain alert. If a lack of alertness is encountered, appropriate countermeasures shall be used. If unexpected fatigue is experienced, a controlled rest procedure, organised by the commander, may be used if workload permits. Controlled rest taken in this way shall not be considered to be part of a rest period for purposes of calculating flight time limitations nor used to justify
any extension of the duty period.





Do any airlines enforce such a rule?

As other answers mention, yes.

EDIT: On the 27th March 2015 EASA has issued the following Safety Information Bulletin :

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.

This still does not mean that 2 people are required, but are recommended.