Hello there,
DOuble pushchairs are always a bit of a struggle as they are simply heavier and larger (as they have to be spacious and sturdy enough to accommodate two kids). I personally would strongly recommend a side-by-side for this as a tandem will be huge and taller, and a side by side, especially a simpler one that only faces forward could have a lower weight.
My personal top suggestion would probably be a Valco Snap Duo which is reasonably spacious for the older one, too, and is not overly expensive. A Joie Aire Twin is very simple and sometimes rattly though is on the budget side of things. And I will also recommend a second-hand Baby Jogger City Mini Double as that's a nice quality double stroller and you'll save money with an older model.