I am so sorry to hear you struggle with this, but a double stroller is always narrower and smaller overall - otherwise, they'd be huge, like impossible to load in a car even when folded. That is why a double is smaller on the seating area overall, and pretty much impossible to accommodate for a 7-month-old and 5-6 years old (size-wise) (and even last a few years). The thing is, weight-wise, that would also strain the stroller's chassis brutally, resulting in a lopsided frame after a few months of regular use.
My advice here is if the BJ City Mini GT2 is too small (and it is one of the largest seats in the double strollers' world), perhaps try to check the Valco Duo Trend, which, I'm afraid, won't be much larger, or if your special needs older child could sit on a bench (without a seat back), check a stroller like the Baby Jogger City Select Lux with the bench accessory (I believe it won't do, but I had to mention this possibility anyway)... And after that buy 2 separate single strollers and use them either separately, or get stroller connectors and connect them (with connectors, you'll need the same strollers, e.g. these connectors from Amazon). A really spacious single pushchairs include the Baby Jogger City Elite 2, the Britax B-Motion 4 Plus, the Valco Snap Trend, or perhaps the Thule Urban Glide or Uppababy Ridge.
P.S. With my older and younger, I often used just one single stroller (for the elder) and an ergonomic carrier or a wrap for the younger. For some cases, it might help quite a lot, having less in your hands to push.