How Can I Use 'Socialization Saves Lives' with a Bonded Cat Pair? There’s a Friendly One and a Fearful one.

Please excuse the length of this.
I need guidance on socializing a bonded pair of cats I’ve looked after the past 2 years. They’ve had food/water every day, insulated shelters in winter, shade in summer. They come running when I knock on my sliding glass door. I rent a townhouse and I can’t bear leaving them behind. I like the clarity and the outlook of Socialization Saves Lives but I’m pretty lost. 

The younger cat, a male I’ve TNR’d, is very friendly with me and my family. He lets us pet him. He swats playfully- he doesn’t seem to know to not use his claws. If approached from outside the usual routine, like the alley or front, he scares and runs away. But he seems to trust me. He‘s quirk(just doesn’t know how to be a pet but who can blame him.)He has curiously wandered into my house a few times and is friendly with my cats. 

The older cat seemed to already have an ear tip (I’ve always assumed she’s female, Idk) was/is very skittish. Early on, she wouldn’t come near while I was outside. She used to only eat well after I was gone. Over time she’s become braver, sometimes waiting outside the door and a few times letting me touch/pat her. She’s still highly wary, esp around others, but has made huge progress. I’ve sat nearby during meals to build trust slowly. I think that’s helped a lot Sometimes she doesn’t eat until the door clicks. Sometimes she starts before I get all the food on the plate (even then she might stop eating and skitter off bc I got up to go back inside)

Recently, I suspect she may have ringworm. I want to get her treated at a vet. If I trap her, I won’t feel comfortable releasing her again. I want to find them both a permanent home, together, they’re clearly bonded. I’ve talked to my county shelter and nearby nonprofits. I know cats need to be adoptable. I want to put the commitment in to make them good candidates. I don’t want to give up on the older cat, I see so much potential in her. I’ve seen her make progress in an uncontrolled environment. I would keep them in a heartbeat but I have 3 cats and I can’t handle 5. 

Could this Socialization Saves Lives work for a bonded pair? Is there a way I can modify it? I emailed them but I’m not sure that they monitor that inbox. Would two side-by-side playpens work? Is it still viable even though I’ve already pet both cats? I apologize for the very long post. I hope the context is helpful. Does anyone have experience or advice?