As Tiggy says, it's a very common misconception that all doubles push the same and that the weight of the pushchair itself makes a difference.
The BJCS seems so perfect on paper that I had geered myself up for paying for a new one and went to John Lewis to try it out to make sure. I was so disappointed about it. I was buying it for my daughter.and niece who were both 10 months and weighed 8.5kg and 10kg resepectively. We put the girls in with my daughter over the front wheels. It just felt a bit heavy to steer but it was almost impossible to lift up onto a step in the showroom. At that small, it really shouldn't have felt heavy at all.
Another thing I'd be put off by is that the baby would be in a carrycot at the front which.really adds to its length and it's not especially short anyway. I think it's best suited if you live in a flattish town/city, don't need to go up and down kerbs often, don't regularly use tight shops or cafes and the older child is likely to stop using it before the youngest is 12 months.
It really depends when you plan to buy as to what I would suggest, purely based on released dates. But it would be between the Britax B-Dual, Uppababy Vista, Peach Blossom 2, Jane Twone and Oyster Max. Only the first two are out now, the third very soon and the others this summer.
I have a B-Dual which is lovely, but feels too wide as a single to be honest. The Vista second seat is very upright so is only suitable for a child who doesn't nap in the pushchair. The PB2 is probably the one I'd go for, especially if you want to buy soon. The Max and Twone may not be as good as they seem, but we won't know either way until they're released.