2 DVD players = 2 different movies (we like to encourage individuality in the kids -- especially when they're tired & cranky at the end of a really long day...). Also, they can actually both watch one movie, with the 2nd DVD unit playing a different movie for my wife through her headphones.
I considered mounting 2 half-din DVD players where the on-star is, but decided my kids are old enough to change & set their own DVDs, and they can reach the headrests while still being seat-belted on during long highway driving.
I got mine from
here $540 shipped (you'd also need to buy the 2-channels IR headphones & 4x4 AV Switcher ~ $700 total. No point to get 2 DVD players if you're not hooking it up to a multi-AV switcher). Very popular item, often sold out.
The one feature lacking was last position memory. If you have very young kids, that means YOU must search & continue the last scene played manually after every gas/food/potty stop, for each kid. Or, you can hook them up to be always-on instead of RAP power.