Supervisor submitted without consent of co-author postdoc. Postdoc also submitted to another venue with consent of supervisor. Plagiarism problem? My previous postdoc supervisor submitted our work to a conference venue in December last year. He was the first author in this submission while I was the second author. It got accepted in March 2020. He did not inform me about this submission. The conference will be held next month; however, they already put this accepted paper online in IEEE Xplore last week. I get to know about this submission when I was searching for some of my papers online in the IEEE Xplore last week.
Now the problem is we have also submitted the same work in April 2020 to another conference (me as the first author, and he as a second author). He was fully aware of that submission, however, at that time, he did not inform me that the work is already accepted in another conference venue in March. Now I know that my work is accepted in one conference while it is in the review at another conference. So there is a clear plagiarism problem.
In this situation, when I informed him, after some argument, he now suggests withdrawing the paper in review with some excuse, and go with the accepted one.
Kindly suggest what should be done? Should I inform everything to both the venues and withdraw from both the places. Other options are to withdraw the accepted paper citing that it was submitted without my consent, or should I go with my supervisor's suggestions? I do not want to face any plagiarism related trouble.
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.