Supervisor submitted without consent of co-author postdoc. Postdoc also submitted to another venue with consent of supervisor. Plagiarism problem?
I cannot really judge any behaviour, but a couple of things popped out for me:

Your advisor submitted an article without letting you know
He put his name as first author
He is now suggesting to keep the one with his name as first author and withdraw the one with your name as first author

I personally would write a letter informing the first conference that the submission was without your consent and ask for their policy. I would then write a second letter to the second conference informing them that the same article was submitted without your consent to another conference and attach the first letter. I would also write a third letter to your advisor with a copy of the first two letters.
Note, this is a &quot;nuclear option&quot; that will likely destroy any relationship with your advisor. But, if your supervisor truly stole your work as you describe, I assume this relationship is already ruined.