We used to go on long trips with a family of 5 in normal sized cars regularly. Of course I would never do that today. Like many families we don't have a car and rent as needed. Long trips with kids are just infinitely much better by train.
Used to be that putting unbelted kids in the back of a vehicle was fine. I grew up using such vehicles. The regulatory environment and equipment have changed significantly over the years. Can't really compare today with the past as trying to live that way will get you seriously fined if not arrested.
https://www.argos.co.uk/product/9020706
Though the ones we have are a bit more bulky.
Of course you won't - the baby seats that are safe enough to pass regulations are too large to fit 2 of them into the back seat of that old normal sized car.
Long family trips in smaller cars were done without baby seats or booster seats.