This has been discussed before.

Communication is crucial.

You need to agree ground rules and honestly discuss what you want and how it will make you feel EG imagine your so kissing another guy, sucking his cock?, swallowing his cum?

Will this freak you out? make you jealous? angry? sad?

and how will she handle seeing your cock in another woman's mouth?

Carefully work out if this is a fantasy or something you both want?

But be warned, experience has told me that one person will be left-out at some point.

Wildeye