Jesse, did you ever carry both kids in your MB (E55?). Do you use a locking clip on the shoulder belts or did you spring for the MB's "baby-smart" car seat?
I'd imagine that a new E55 would have instructions and/or provisions for a car seat in all three of the back seating positions, at least until they add rear side air bags or that airbag curtain...
Robert W. Roe
1984 300SD 165,860 miles