Yes, it is possible to fit an infant car seat and a sports seat at the same time, i.e. for a baby and a toddler. The pushchair is compatible with infant car seats that can be attached with a separately available universal adapter, while the other seat is used as a sports seat. This means you can attach an infant car seat (e.g. from Hauck Drive N Care or other brands such as Cybex, Kiddy, Joie) to the frame and use the second seat as a sports seat.
Yes, that is possible, but only to a limited extent. We actually always have the tub at the bottom and the sport seat at the top facing backwards.
The upper tub is only possible if the lower sports seat is rear-facing, which means that the child in the sports seat has the tub in front of its face, which is not very exciting.
Tub down and sport seat up forward facing also works, but if the big one drops something, it lands on the baby in the tub, not very great either.