My thoughts would be to buy 2 single units with straps to attach to the headrests.
I got one for my daughter a few years ago & I wish I'd done it many years before.
On the initial Tuff Truck return (driving 11hrs + on monday) all I heard from her was, Laughter from the funny bits,
Dad- I'm hungry or Dad - I need to go to the toilet.
Much, much better than the usual stuff kids ask when travelling
Pro 1- they vary rarely will want to watch the same movie at the same time
Pro 2- if needed they can be removed easily for viewing elsewhere.
Pro 3- Price varies but I suspect cost would be less than $5-600 for a couple of decent brand named units.
good luck with what you choose!
cheers