Walk in walk out really does focus on knowing the different cries, and yes you would only respond if he sounds like he truly needs you. All other cries or sounds you would leave him to work it out himself. It does seem confusing and often people think it is the same as CIO, but it's not because you would never leave him to cry if he were distressed. Success really does depend on knowing the different cries though. If he is really upset then you would comfort as necessary. *Technically* you do this with him still in the crib and with as little contact as possible, but TBH I think you have to do what works for you and him, so if that includes picking him up briefly, or sshing and patting, or rubbing his back, that is fine. Whatever he responds to.
I have no experience with LOs sharing a room, so I'm not sure if you can do it while they are both in the same room, but I'm sure that some parents do sleep train one LO in the same room as another.
The most important thing is whatever plan you choose, you follow through. So pick which room you will sleep train in (yours or DD's), pick which method you will use (GW or WIWO), and make sure that from that point you are consistent. You can't stick to your plan one night but then DH lets him in bed the next, it sends mixed messages. Sometimes DH's need a little talking to first so you are on the same page!