I thought hard and long about putting a Kinder seat in my car so I could carry both kids at once. I even went so far to start cutting wood panels to make the seat and I bought a seat belt to fit. Then the issue of safety came up and I started to find more and more people who took them out just to remove the temptation to put their kid back there. A small rear ender could have devastating results.
My kids take turns in "Dad's white princess" one at a time and it has worked out fine. A bit of "one on one" is always good.