From a legal standpoint, the requirements are
ORO.FC.235 Pilot qualification to operate in either pilot’s seat
(a)
Commanders whose duties require them to operate in either pilot seat and carry out the duties of a co-pilot, or commanders required to conduct training or checking duties, shall complete additional training and checking as specified in the operations manual. The check may be conducted together with the operator proficiency check prescribed in ORO.FC.230(b).
(b)
The additional training and checking shall include at least the following:
(1)
an engine failure during take-off;
(2)
a one-engine-inoperative approach and go-around; and
(3)
a one-engine-inoperative landing.
I guess any extras, over and above this is dependant on company procdures, as is demonstrated with the monitored approach in RYR for LVOs. Other restrictions like being PM only may come from other factors like insurance requirements