For my own tastes I am very selfish and couldn't share fantassy with another, but I do know a relationship where it works very effectively and has continued for years.
She is married to her Dom and also has an online Dom. The 2 Doms know each other and communicate so that there is no clash in ongoing tasks.
The only thing I found strange in this relationship was that its the online Dom that is most into the more physically side of D/s, and the r/l Dom that takes the mind/mental approach. I would have sorta expected it to be the other way round.
Brosco