From Kindlings:
Would you want to be trained (in whatever areas applied) by someone who you were not owned by? Why or why not? In what areas?
Yes I think I would like to be trained by someone other than my owner in a few areas. The only reason I would is because I know my Master doesn’t have the knowledge or training in this area. One of the areas I would like to be better at is food and beverage service. Another would be submissive positions.
I’m a large, curvy, lustful submissive learning the ropes in a full time D/s relationship. My explorations with love and life and BDSM have brought me closer to the edge of pain and pleasure, deep desires for rope bondage and debauchery but moreso a desire to delve deeper into submission and service to my Master 
I think I would be ok with being trained by someone else but would depend on what it was how comfortable I was with it … although, of course, if my Master decided it was going to happen, it would happen regardless of how I felt about it.
His angel
From the opposite perspective, I would never train anybody in anything unless two conditions are met:
1) I absolutely know what I’m talking about.
2) There exists a mutual trust between master and pupil.
Simple as that…
Good point Sir.