I've done this plenty, mostly MFM not D/s related, but two ways that worked for me in that way -
Once with a subby guy, he made me mini-Dom, which I am uncomfortable with, sort of middle management, lol.
And one glorious time, both guys collaborating and using me.
I think the sharing -as-dominance can be tricky, in a relationship. But if she likes MFM and you give it to her as a treat, that's so nice. Confirms she is slutty, so you can tease her, and you get what you want. That's what we did more, the actual sex was more equal, or me asking for things.