That’s great! I hope all works out with the Lionhead girl! ((((successful spay vibes to her!))))
Are you saying in terms of car bonding just around the neighborhood, or are you thinking of putting them together on the car ride home from the rescue? She will still need to heal up for 3-4 weeks before they are put together. Until that time, you can switch pens each evening, make sure they are in close proximity to each other, but cannot actually reach each other.
I never did car bonding myself, we took rides on the washing machine. What you want to do in the car would be put them in a box or laundry basket on the back seat, something that you can have easy access into. Someone else would need to drive you, you’d be in the backseat with them watching out for any scuffles. It would be a good idea to have an additional carrier, just in case they need to be separated in transit.
What I liked about the washing machine was that straight from there we could go to our neutral bonding space with little in between time. You might try that first to see if it works well enough.