Resident Cat and New Cat Advice : Pets

Hey all, I’ve been introducing my new cat to my resident cat for about a month now. For the most part I let them roam around my apartment and they more or less keep to themselves now, a stare down every now and then but they haven’t fought or anything in about 2 weeks. Anyway, I decided to see about letting them both be out last night, and the resident cat decided to corner the new cat under my bed and one of them was growling (I’m not sure which). I was able to separate them before anything went down.
I was wondering if anyone has any advice to assist them in getting used to one another or if it’s just one of those “it’ll happen with time” situations.
Any help would be appreciated, thank you!

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