I strap them in like children too, in the back seats. The open fronts face inside so the girls can look at each other, and so I can see them in the rearview mirror. The shoulder restraint keeps the carriers (hard) from shifting or tumbling if I have to brake suddenly. This has happened to me, and the carriers stayed put. I pull the part of the seatbelt that goes around the waist tight right under the place where the top and bottom lock together.
For those who run the seatbelt through the handles, the carriers don't shift around if you have to brake hard or make a sudden turn? If not, maybe I'll try that. It sounds easier than fussing with the position of the seatbelt.