Thats a tough shoot. If you have a 1.8 50mm I would say use that, if not use a lens with a wide aperature so you can use a faster shutter speed. Also make sure both cars are going the same speed. A flash would work as well. You may need a high ISO depending on the light. I am interested to see the results you may get some interesting shots. If your giogn to have a longer shutter speed even 1/2 a second I would suggest using a beanbag on top of the window to rest the camera on to reduce camera shake, just make sure not to drop your camera out the window. I have seen an interesting set up on this site of someone who mounted a tripod on the front of their car and positioned the camera to look back at the car. I would not drive too fast in this situation but that may produce nice results as well. Dont worry about the speed of your car because any motino at all will make the car appear the be moving. Also make sure the road is nice and smooth, maybe look for a newly paved road or parking lot.