If tableField = "Process ID" and layerField = "PID" then this works:
IRelationshipClass relationshipClass = memoryRelationshipFactory.Open( "Join", table as IObjectClass, tableField, geoFeatureLayer.DisplayFeatureClass as IObjectClass, layerField, "forward", "backward", esriRelCardinality.esriRelCardinalityOneToOne );
If tableField = "Process ID, Year Made" and layerField = "PID, YM" then it fails.
(Table is SQL Server table.)
I have tried brackets and quotes to no effect. Is it possible to pass a compound key?