What can I do to prepare my dog for a long car trip with another dog?

Practice and planning.

Practice good car manners now. If they aren't good passengers, now is the time to change that.

Plan your route. There are rest stops and dog friendly places to stop where they will have room to run and play and "do their business". Look for guide books in the pet section of the bookstore or library. "Dog Lovers Guide to the USA" is a good one.

Another tip.

Have ID tags and microchip information updated. Pets can freak and bolt or pull a "Homeward Bound" and return home. If your pets suddenly dissapear for any reason, you want the best shot at getting them back.

If your dogs are familiar with each other and get along, they should be fine. However, if they might fight, purchase dog crates and have them ride in those. The dog crate is the safest way to protect your dog in the car.

Some dogs ride better outside of a crate, and if that's the case for yours, buy "dog seat belts" and use those to keep the dogs from jumping on each other. If possible have one dog ride in front and one in back so they won't scuffle.

is this a dog your dog is familiar with? if not i would see if you can find some kinda contraption to seperate the back seat so they each have their own side. otherwise, just get some toys for them to play with in the car, get something that will hold on the door for water and food. and make sure you stop every few hours for them to go potty.

You can start showing your dog to other dogs before the trip.

The dogs should be in crates or some type of restraints.

Pet smart has doggie seat belts.

my aunt takers her dog to florida every year... and a dog cant stad that long of a trip so go to your local vet and ask them for these pills that will help Her/Im

try socializing them together, so they get to know eachother before they are put in a confined space, hopefully they should settle quicker this way?