Poor little lass. Every single dog is different and has its own unique personality, traits and behaviour shaped by its initial start to life so for that reason it's impossible to give anything specific to your dog. From a general standpoint however and based on my own experience with very anxious, fearful dogs I'll offer a few pointers.
It's not at all uncommon for rescue dogs to suddenly decide they don't want to leave a person, the house or go beyond the garden / immediate vicinity mainly because they'll have settled in to their new home and latched on to the place or person they feel most secure and safe with if you will. That's a good thing because it means you've established a relationship to the point they trust you and feel safe and more at ease around you but it's also sometimes a really key and crucial point where they can go one way or another.
It's understandable owners want to do everything to keep the dogs getting more anxious by giving as much reassurance and comfort as they can but it often does more harm than good.
My advice is to take small steps by for example taking her out on a literal one or two minute walk with the other dog to help encourage her outside and let her watch and learn from example.
Putting her on a lead to take the small walks is crucial to keep her bolting. If she stops, plants her feet or starts to struggle or whine, a single firm but gentle word of reassurance and tapping on your leg “Hey hey it's OK come here” and then say nothing else. I don't turn back and head for home whilst a dog is still in fretting mode because it just teaches them the easiest, quickest way to get back is to create and kick up a fuss. Once they're following and walking alongside without too much whining or fretting, I turn round, maybe give one “Yeee good girl c'mon let's go back this way” and head for home.
Getting her favourite toy or whatever else she loves and throwing that into the mix will help her to form a new association with the environment. For example we had one who was absolutely petrified of going in the car but obsessed with tennis balls. I left both back doors of the car wide open, popped a tennis ball on the seat then pottered around like I wasn't looking. His curiosity and need to get hold of the ball made him sniff and mooch closer until he was hopping straight up into the back. Then I opened the front doors, hopped in the driver seat and tapped the passenger side “You coming up?” and he jumped up, sniffed around and got back out again so once we'd reached the stage he happily got inside the car by himself, we drove just up the road and back again to get him familiar with the sensation. I then drove just a little further and jumped out to let the dogs play in nearby woodland which is one of his favourite things so he quickly started to make a new association with going in the car to going places he loves and playing with tennis balls.
She will pick up on whatever vibes you're giving out and if she senses you're worried or if you give excessive reassurance, she's gonna go “Oh God what's wrong? I can sense worry what is it I need to be scared of that they're scared of??!!”
Nonchalant and flippant almost so she starts to think “Oh.. oh well they're not too worried and the other dog isn't bothered either maybe it's OK” It's amazing how many dogs we've had short term foster placements that were in similar shape and tended to stick to me like glue despite my attitude being nonchalant, flippant and almost like I hadn't noticed their worries and couldn't care less.
The fear of people can be a tricky one and does need more time but again it's a similar thing of baby steps and letting her get comfortable with people without them coming to approach her and getting her even more up the pole.
Ideally a very select few people she can get used to seeing out and about or just being near her without them directing any attention towards or physically touching or trying to will help her start getting over the initial anxiety or worry about what they might do.
I can't emphasise enough how important it is to make sure the trainer or whoever is coming to work with her understands that, doesn't impose or enforce herself and ideally is experienced and familiar with very anxious and fearful dogs.
You're essentially teaching your dog to form new associations with things she's fearful of right now and your other dog is an ideal way to demonstrate the relationship between you and the outside world. Keep everything light and make everything a super happy fun time and if she's not force or pressured to get involved, chances are she'll want to be involved!
Good luck 