Upvote:2
No person can grant a partner a stay. What happens is that when one partner is an EU citizen, and another partner comes from a non-EU country, the non-EU partner can request a family reunification visa which should be granted by the visa officials unless there are doubts about the relationship.
That sounds like talking about words, but it matters who does what.
A marriage is not necessary for a family reunification if there appears to be a stable, long-term, well-documented partnership. But if one partner is married to a third person, that puts the other partnership into doubt. Perhaps it might work if there are divorce proceedings going on, but it sounds highly questionable.
You would have to talk to an immigration lawyer, but I don't think it will work.