Roman Catholic excommunication?

I was baptized and confirmed in two different churches (two different states) so to officially not be a member of the religion/not count towards the number of Catholics can I just send my "excommunication letter", as I like to call it ,to the church I was baptized in or do I have to send it to both?

Thank you.

Send a letter to the bishop of the diocese where you were last an actively "registered" Catholic and the parish where you were baptized. Include that you have joined a Chucho and abandoned Catholic beliefs. You have been automatically excommunicated already by Canon Law. The bishop's letter is only an official affirmation of this.

Denying any of the Church's laws will excommunicate you. You have an abortion, assist someone else with a grave sin, etc. you automatically excommunicate yourself. Denying the Lord, committing blaspheming, etc. You don't write a letter to the church to kick yourself out. Just sin and you're out.

Just light a bag of poo on their stoop

I'm going to assume that you are writing an official letter to the Catholic Church resigning from it, right? Never heard of this being done before, but legally, if you wish to break a contract (and that is what a Catholic has with the church if you are baptized and confirmed), I would get a copy of my Baptismal Certificate (from the parish where you were Baptized) and a copy of my Confirmation Certificate and I would write a formal letter to the arch diocese wherein you now live and address it directly to the Bishop there.
Send it certified mail with a return receipt. Keep copies of everything you send. I think you are officially done...resigned...excommunicated...done with it.

Send it to both; that way, nobody can quibble.