When our twins were babies, 8 months, they would travel side by side in car seats on dinette faceing forward. I had to buy longer seat belts to use with car seats. They had the use of the table
an could talk an hold hands as we traveled down the road, they thought that was great fun. As they got older they sat in same spot having fun an talking, you know how girls are.
If they were in separate seats I think they would have been constantly looking for one another
and you would know what kind of ride that would be
If you have two that don't get along with one another
you could put a large pillow between them. You could also use large pillows if your concerned about side impacts. The twins are now 17 an they still travel in the same spot, when there with us.