What do you mean by free roam together and doing bonding sessions in smaller places? You can’t just put them together and then take them elsewhere to work out their issues. Can you give them each a room while you’re letting bun #2 settle in and then move bun #2 into the same room with your girl and divide the room in half? You need neutral territory while you’re bonding them anyways, so it’ll really be for the best if they aren’t totally free roaming while you’re attempting sessions.
So let bun #2 settle in for at least a month away from your other bun before you prebond, this will help her to not feel overwhelmed and stressed from so many new changes and then smelling a new potential friend or foe. After a month of letting her settle in, swap their items around to get them used to each other. After a month of this, then you can begin short bonding sessions in completely neutral territory and increase the sessions based on how well they do together. To make neutral territory, you can spritz white vinegar over the area that you plan on using. I suggest having one room set aside that they take turns having play time in that way you have an area for semi-neutral area when you’re ready for that step.
I know you say she’s a sweetheart, but females can be aggressive and territorial when bonding, especially because she already thinks everything is her. So just warning 