I need a double stroller (umbrella) with the longest possible backrest for a long toddler? What do you recommend?
I would first suggest looking at this article about umbrella strollers and the reasons why I think it might just get too wobbly and unnecessarily long after folding, and them, this article about double strollers just to understand the mechanics and what can work best there. That would be the start as when choosing the right stroller; you need to have and understand your stroller priorities as well as dangers and disadvantages of the chosen pram or pushchair.
Next in line, if you truly know that an umbrella type of stroller is what you need, I will recommend a few that have taller seatbacks that should work. Do bear in mind that the seat's width and maneuverability are also important and will be worse with a double stroller. You don't mention your stroller budget so I will not take the price tag into consideration much, but I am going to try and keep it in the doable price range nonetheless.
The first that came to mind is an umbrella buggy that is simple yet functional, and the height of the backrests is good for a toddler: the Koelstra Simba Twin T4. Truly, here, the height of the seat back will be absolutely worth getting the model, if nothing else will.
I will mention mid-size double strollers that are also suitable and not overly heavy - and I believe would be much more sturdy while much more pleasant to push thanks to the single handlebare of the stroller. That would be the Baby Jogger City Mini Double2, Valco Snap Duo, Baby Monsters Easy Twin, Peg-Pérego Book For Two and similar.
Hope I provided enough inspiration! Good luck to you and your family, stay safe 😊