I think the youngest needs to go in the Peach Blossom 2 until 6 months and then you, or the child in our case, can choose who goes where.
Access isn't the best, but the ease of se, narrow size, neat fold etc carry a lot of weight. I don't have any problems putting my two into the lower seat of the Peach Blossom and when I remember I load the lower one in first before putting the top seat on.
The cons of the other two are that the Donkey is a piglet if you need to convert it. Sadly, I do, and it doesn't get any easier. I long for a huge hall to push it in in double mode and leave it up, and a car I can lift it in open. Folded its large, doesn't lock shut. It slides to the floor open if you don't fix the front wheels which then means you forget when you open and wonder why you can't move easily! It's got lovely big seats though. Looks gorgeous, is easy to push - but I wish my hand naturally went on the foam than the extension. The basket is shallow with the bar in the way - I can get more under the Peach.
I haven't properly used the BJCMGTD - I have seen and played several times, i have had the single GT and double BJCM and Elite and the GT seems to be a good compromise between the two. I woud certainly consider it for ease of use, good push, etc but it's a big chunk to lift into the car each time and the children are forward facing - but with the Peach I can hear my eldest chatting from underneath. With the double BJs, I had to keep running round to hear. But it's much easier to flick a pushchair out and go than mess with adaptors and seats.
I should add that the Jane Twone seems easy to push, small to fold etc too. I also love the Stroll Air MyDuo. I had the Aussie version and loved it. Well worth a look for easy to use plus it has air tyre options.