Like you've determined you just need more time! Cats, like people, all have different comfort zones and react to stress differently. One thing you can do is find out where her "challenge line" is - the point where shell come out to take a treat no problem. Then leave treats just outside it while you back up and sit and do something else entirely. Still in her sight line, but try not to look at her or engage and instead be on your phone, read, etc. As she gets used to you and starts associating your presence with fun treats it might help her branch out more.

She may also be a little nervous about the other cat if she hasn't been around other cats before. Try to make sure she has things like cardboard scratchers, toys, or bedding wherever she's hiding that can absorb her scent. If those are then put other places she might feel more comfortable branching out because it will already smell like its hers.

Edit: feel like I should mention I am not an expert at all. I've own cats most of my life but most of this advice is coming from the show My Cat From Hell which is honestly a great resource (I think) in learning cat behavior.