We use ours on long trips all the time!!! They are WONDERFUL!! Most portable DVD players have two headphone jacks. Don't buy one that doesn't. You can buy the headrest attachment things at Wal mart, although we don't use one. It makes it so that only one child can see the screen properly. So unless you can get a "middle of the car roof" mount, which can be really expensive the headrest thing might cause more fights than it's worth! We just use a square plastic bucket thingie down in the middle of the footwell between the kids. My husband has a piece of flexible rubbery stuff that he puts on top of the bucket so that when the DVD player is on it, it isn't sliding all over the place. You can also get velcro for that purpose. If you can, try and get a dvd player with a long battery life. Most only last about 2.5 hours and that won't get you very far! (if you are on a plane that is...most come with lighter/charger thing for cars so the battery doesn't die!)
We have a Sony. It's has a four hour battery life. It was on the pricier side, but it's been well worth it. I will say that before the Sony we had a random brand and it lasted for a long long time and worked just fine. As long as you two headphone jacks you'll be good to go!
Remember to get a full charge on the battery before the long trip!!
good luck, have fun!!!