Double stroller/pram

I hope I can describe what I need 😩

So in October we are expecting new baby.My oldest will be 21 months old.So I’m looking for double stroller.One part to have regular seat for him and another one to have pram but later can be taken of so I can put regular seat.

And when my oldest outgrow stroller I can take his seat so it can be like regular stroller.

I ask maybe impossible but I didn’t see something like that 😩 and I hope I describe what I want 😬