Being Dutch, I can assure you these Dutch tandem are horrible. They steer like a barge, and are imposible to get up any hill. I think a childback tandem like the Thorn Voyager is a much better idea (we ha done, and it was a great bike). For the first year or so you could have your older child on the back of the tandem, and the younger perhaps in a second hand trailer. When the younger will no longer fit inside the trailer, your older will be old enough to ride her own bike.
I dont like tag alongs. The ones I have tried were all pretty unstable. I have not tried the Pashley.
Willem