public void Merge( 
   Layer destinationLayer,
   Layer sourceLayer,
   IEnumerable<long> sourceOIDs,
   Inspector inspector
)

var mergeFeatures = new EditOperation();
mergeFeatures.Name = "Merge Features";

//Merge three features into a new feature using defaults
//defined in the current template
//At 2.x -
//mergeFeatures.Merge(this.CurrentTemplate as EditingFeatureTemplate, featureLayer, new List<long>() { 10, 96, 12 });
mergeFeatures.Merge(this.CurrentTemplate as EditingRowTemplate, featureLayer, new List<long>() { 10, 96, 12 });

//Merge three features into a new feature in the destination layer
mergeFeatures.Merge(destinationLayer, featureLayer, new List<long>() { 10, 96, 12 });

//Use an inspector to set the new attributes of the merged feature
var inspector = new Inspector();
inspector.Load(featureLayer, oid);//base attributes on an existing feature
//change attributes for the new feature
inspector["NAME"] = "New name";
inspector["DESCRIPTION"] = "New description";

//Merge features into a new feature in the same layer using the
//defaults set in the inspector
mergeFeatures.Merge(featureLayer, new List<long>() { 10, 96, 12 }, inspector);

//Execute to execute the operation
//Must be called within QueuedTask.Run
if (!mergeFeatures.IsEmpty)
{
  var result = mergeFeatures.Execute(); //Execute and ExecuteAsync will return true if the operation was successful and false if not
}

//or use async flavor
//await mergeFeatures.ExecuteAsync();